Should be doable with Termux:

1. https://f-droid.org/packages/com.termux (the terminal emulator itself);
2. https://f-droid.org/packages/com.termux.api (for termux-sms-list and termux-sms-send commands);
3. https://f-droid.org/packages/com.termux.boot (to make the script launch automatically after reboots; don’t forget a wake lock to keep the process alive).

termux-sms-list returns messages in JSON, which is easy enough to handle with, say, jq in bash or json in python. The script itself can be a simple loop that fetches the latest messages every few minutes, filters for unprocessed ones from whitelisted numbers and calls termux-sms-send.

Maybe it’d make sense to daemonise the script and launch it via sv.

But the Termux app weighs quite a bit itself.

Like Firefox ScreenshotGo? (I think it only supports English though)

I’ve never encountered a keyboard app with UI/UX comparable to Fleksy, so that’s what I use (and UI/UX is everything for a keyboard).

The settings became a bit silly in terms of UI in the course of updates though, I mean specifically the keyboard itself.

Does it? I set its $PREFIX/etc/resolv.conf to Cloudflare and dig uses it fine.

I don’t usually edit PDFs on my phone. On the PC, I use pdftk+qpdf+img2pdf+ocrmypdf (all command-line apps). Some of those can be found in the default Termux repos once you install the terminal emulator; some, perhaps, could be compiled and used as well.

Some updates might be restricted to certain architectures, Android versions &c. Some could be beta versions. Or your repositories simply need to be synchronised.

If it isn’t the latter, check the following settings: “Include incompatible versions”, “Include anti-feature apps” and “Unstable updates”.