After trying this addon a few weeks ago, I kept tabs on its development, because I liked the style and the responsiveness of its developer. After seeing how much features were added in that short time I wanted to add my 2 cents as an inventory power user.

First of all I really liked the addition of collapsible categories as well as the addition of per category grouping, those are features I missed when I first tried it. The newly added "Help" button is also a great addition. But there are some crucial features still missing.

Oh and please don't take this the wrong way, I don't mean to be overly critical or mean about it, even though it might sound like it. I appreciate your work and want to be helpful by showing how I would like to use this addon.

Which features are still missing?

Debug view

I think it would be helpful to implement an optional view, as an addition in the tooltip (or somewhere else), that shows metadata information for each item. Which search terms (keywords) are associated with the selected item, which fields of the items are searchable, so you can find the best search terms or the reason a search term fails. This could be shown with the press of a button or toggleable.

Specifying your search

Just like google, your approach was to make search simple, which is a good starting point ... and for some that's enough. For power users you should consider adding parameters to your search. What I mean by that is something like name: cloth searches only in item names, type: tradeskill searches only in the item type, subtype: cloth only in the item subtype, tooltip: to increase your standing searches only in the tooltip and so on. There are more terms possible, you know better than I do which could be sensibly used.
Now I know that you can use the exact operator #, but that is only true, if you (and Blizzard) are consistent with type and subtype naming. Now I don't know about Blizzard, but you already have at least one problem there. You used the name reagent for reagents (tradeskill-items), but Blizzard also uses the name reagent (itemclass-id: 5), but not for reagents, those are in the itemclass tradeskill. Now if you implement a search for specific keywords, where a user can specify if he wants to group itemclass reagent instead of keyword reagent that would still be confusing to some degree, but at least its a solution to the problem. I'm curious why you implemented it that way, because according to your own guide you used all of the item types (item classes) to tag your items (well except for reagent obviously, the keyword exists but for tradeskill items), added some shortcuts (equipment, gear for weapon & armor) and added some sensible ones that aren't that well implemented by Blizzard, like toys and mounts.

Why am I talking about this in such detail. Well because I couldn't figure out how to keep Abrasive Polishing Cloth out of my reagent&cloth group for the longest time. I know now that I can do it with the exact-operator (your help menu was really helpful there), but in the beginning this was a major turn off. I also couldn't figure out how to target Abrasive Polishing Cloth, that's why I suggest the Debug view. I tried searching for everything in that tooltip, but it can't be searched, no matter what I tried. The official subtype is finishing reagents, but that keyword isn't implemented yet. One of the few subtypes that aren't in it seems.

Similarly creating categories for pets isn't that straightforward either. Using #battle pets only found caged pets. Next I tried #battle pet, which works for both new pets and caged pets. Neither worked for relevant pet items, like Training Stones or Battle Pet Bandage. Removing the exact operator included the stones and bandages, because it is part of the tooltip, but still Polished Pet Charm was still not part of that category. So using only pet I got my desired result, but I don't feel sure at all that this category won't backfire as soon as I carry an item with pet in its name or tooltip without it being a pet. I haven't tested it with mount, but if it were similar, any item with amount in its tooltip would be categorized as mount (This should make it clear why I have a problem with using pet as search term). There also seems no way to include items, that are used to create pets, like Rock of Aegis. That's where the next item on my wishlist comes in even though I could elaborate further on the topic of Specifying your search.

Creating categories based on item IDs

This has been mentioned before and I cannot emphasize it enough: This is a must have feature. There needs to be a box, where you can paste a string of comma-separated item IDs and those item IDs are then part of that category. They need to be visible, so you can always delete them again (all at once or just one) if necessary. It is really easy right now to get lists of item IDs for lots of use cases. The one that tripped me up were reputaton items. If you search for reputation you will only find items that have reputation in their tooltip, but not every reputation item has that keyword in its tooltip. You can add Dragon isles artifacts and might think that gets you all item, but noooo Titan Relic has no subheading like that. So you get to play "Whac-A-Mole" all the time. Here's a list of all dragonflight reputation items: 191251,193201,191264,191255,199906,200450,200071,200449,192055,200443,200093,200447,201411,201412,201991,200224,200452,200285,201921,202091,200287,200453,201922,202092,200288,200454,201923,202094,200289,200455,202093,201924,205365,205342,205985,210422,210419,210420,211417,210421,211416,210423,198790,201782,201783,201779,201781,205251,206006,205253,205250,205254,205252,205249,205992,205991,205998,205989,210921,211370,211369,210958,211131,211366,211371,210916,210920,211372,210950,210730,210959,211353,210957,210757,210952,210997,210954,210847,210951. It took me less than 60 seconds to create that list from a website that had it curated in a more legible format.

The same goes for putting your items manually into a category. First of all you need to have the item in your inventory. This isn't a bad thing at all, it's a solid foundation. But having a way to batch-import items you don't own yet, by using an item list from the TSM website for example, that would be great. Right now there is also no way to see which items were added manually (only if you export a category) and no way to remove them once they're out of your inventory (except by exporting the category, removing the entries and importing again). Second of all, the grouping functionality doesn't work for manually added items.

Those items on the left were added manually, so they aren't grouped by expansion like the rest.

Now you might be able to implement a "reputation" keyword faster than the item ID importer, but that would just move the goalpost to the next thing. If you know where to look, you get great item lists for every scenario imaginable.

Fix search bar glitches

This is just something rare I noticed during testing, but I wasn't able to find the reason for it and make it easily repeatable. Sometimes the search term in the inventory search bar produces a different result than the search term in the categories search bar. It happened e.g. when I tested out how to deal with pets. After using a lot of different search terms in the inventory frame and writing the paragraph about it above, I tried it again with the exact same search terms in the categories search bar and got different results. Even when I used the same term in both fields I got different results (sometimes it highlighted more items in inventory, sometimes fewer). But when I tried to replicate it right now (for a screenshot), it worked perfectly.

Can't respond to everything just yet. But for the debug view run /bgr c debug_keywords true

Concerning the last point about the search bar / category search.

Yesterday I posted about this bug in the Discord's Support channel, I'll put the details here to help with debugging.

I experienced the issue with this item:

In my example, the search string I was using was jewelcrafting&#profession tool.

But, I think I identified the problematic word to be jewelcrafting.

When using it in the search bar, it would return the item correctly, but when used as a category search, it wouldn't.

The item would stay in the automatic category "other" instead of in my manual category "Jewelcrafting" which is positioned above "other" in the list and also has a higher priority (Normal, vs Low)

This item doesn't have the keyword jewelcrafting, so I'm expecting that it searches using the text in the tooltip (which it does with the search bar only).

Let me know if you want more details.

Update: I'll be adding #finishing reagents and fixing the #battle pet, #battle pets issue.

I think everything described here has been resolved now.