Instead of having to come up with 5 different keybinds for each of those, you should only have to come up with one. Holding down the button and scrolling should switch between the different placement modes (including vanilla placement mode).

Alternatively, another solution could be to toggle the different modes rather than having to hold down a button to use them. It can get pretty annoying having to constantly hold down a button while walking around and placing blocks using these placement modes. I don't have enough fingers for these keybinds. My forefinger, my middle finger, and my ring finger need to be on WASD, and my pinkie needs to be on shift, and my thumb needs to be on space. That leaves no finger available to continuously hold down a button for special placement modes. Especially if I need to activate multiple placement modes at once. I can't put them on my mouse, because my mouse only has 5 buttons, which I've reserved for attack, pick block, use item, sprint, and toggle perspective.

At this point it's easier for me to just go into the settings and manually toggle the placement modes on by changing their setting to allow for an empty keybind. Changing it to not allow the empty keybind toggles the placement mode off. It's a really clunky solution, especially for flexible block placements, because I have to navigate partway down in the hotkeys GUI to find them every time I want to toggle them on or off.

These will likely get something like you suggested.

But I'd say for the majority of users any toggle-type functionality will be a lot more clunky to use than the current solution. I would personally (almost?) never use a toggle-type mode selection, as it's far slower to get the mode you want with something like that, and in most cases when building some redstone stuff you need to change the rotation mode for pretty much every block you place. When I'm building something, I don't need to have full maneuver control and sprint + sneak ability concurrently, so I can use my pinky and thumb and also index finger for the flexible/accurate controls as needed. Now granted sometimes it does mean I need to strafe using my middle finger or use other "non-usual" controls...

There is currently also the feature "remember flexible from click", which is at least supposed to remember the flexible placement setting from the initial right click for as long as you hold down right click while using the Fast Block Placement feature. So if you are placing lots of the same block in the same orientation, then you shouldn't have to keep holding any flexible placement keys, assuming you are able to use fast block placement (i.e. the blocks are in a row or something). I think this could also be expanded to at least also cover the accurate placement feature, and maybe also to something like "use last placement" key or persistent tweak.

There is definitely lots of room for improvements here, but the current individual controls are never going away, that's for sure.