Make it possible to write a description to the schematic, to allow you to write, for example, how many stacks of items are present in a box or hopper (as for a timer).

Came to add a feature request for a text field/description, found this one already open, so I'll just add a +1 for a text/description editor.

This has been intended from the very beginning, but I just haven't added a GUI for editing the description text yet. The schematic files have already had a field for storing that text since the very beginning.

Also as for the amount of items in inventories, if the mod has access to the inventory data when saving the schematic (which currently only is possible when saving the schematic in single player), then the schematic will have all the block entity NBT data and thus the mod will have access to the inventory contents in the schematic. And in such a case you can use the Block Info Overlay to se the inventory contents in the schematic. I think the default hotkey to show that is i, so when you hold that and look at the blocks in the schematic it will show you an overlay including the inventory contents.

I was not aware of it, thanks for the immediate and correct answer! Will the ability to add a description be possible in future updates?

Yes, it will likely be part of the next bigger updates I'm currently working towards on 1.12.2. No reliable ETA for that yet, all my previous estimates have just been slipping by...