I’m not sure where ConnectBot stores its keys, so I can’t help with the ‘transferring to termux’ part :-)

What I can tell you is that you should be able to generate and use SSH keys directly in Termux. Here’s a quick guide on how to do it: https://gist.github.com/evandrocoan/f503188587587d7b1d1ba8746c9c6107

I’ve switched phones recently, and I haven’t set up my keys yet, but as far as I recall I went through a similar process a few years ago to set up SSH keys on my phone (I haven’t tried the steps in that article myself).

because sometimes you need to investiagte an issue that happens only on the production machines, and you can’t/shouldn’t setup debugging on those.

I’ve used Arch on an RPi 2 back in the day. It works just fine, and fun to play with. But at the end of the day, you don’t want things changing every day on a server. I’d recommend going with Raspberry Pi OS; it’s relatively light, well maintained, and just works.