Hackers Keyboard has that via ctrl+z, but only in horizontal mode. For this only i had my Galaxy S3 in horizontal per default. But can't do that with modern phones' aspect ratio and i'm missing undo since then.
Ah, that's the "Android navigation bar". All I know is that Google really doesn't want that bar to get hidden when the keyboard is open, as there's otherwise no guaranteed way of closing the keyboard. So, I'm guessing, you'd need some extremely hacky methods which modify the OS, like XPosed or whatever is the current thing is in that regard...
So, uh, after writing that, I realized the most recent FlorisBoard update actually removed quite a few features and this might not be true anymore. 😅
For example, FlorisBoard had glide typing built-in, now it doesn't anymore. Apparently, the dev is working on re-implementing it along with word suggestions (which are currently missing, too).
I actually happen to not need these features, so I kind of forgot. And this most recent update came with an add-on store integration, which seems like it could become quite cool in the future, when there's more add-ons: https://beta.addons.florisboard.org/
Aside from that, I think, FlorisBoard still has a few more configuration options, but on the other hand, HeliBoard has a Kaomoji tab in its emoji selector, so I guess, I'll give HeliBoard another shot. ¯\_(ツ)_/¯