Unexpected Keyboard works well for me when using Termux, possibly even better than Hacker's Keyboard, since I find it easier to swipe on a key to get to uncommonly-used symbols rather than switching to a different keyboard layer. Every now and then I accidentally swipe a key when I meant to press it, and end up entering a accented character when I didn't mean to, but this is fairly rare. I don't use Termux very often, but for occasional vim or terminal usage it's totally sufficient.
One cool feature of Unexpected Keyboard (which may be available elsewhere, I haven't looked at many others) is that you can swipe left and right on the space bar to quickly and accurately scroll left and right in a text field. I find this about as fast as tapping at a position in a text field, but much more accurate.