I'd like to use Termux for git/compiling/executing, but prefer external Android editors for editing source code.

The obvious approach, of doing git init or git clone in /sdcard fails with:
error: chmod on /storage/emulated/0/reponame/.git/config.lock failed: Operation not permitted
(see https://plus.google.com/+LarsStokholm/posts/eZqMLNmKX1n and similar issue corbinlc/gnuroot#55). It would be great to find a workaround for that, but it seems tricky.

So instead, how about making Termux's app-private storage available to other apps via a Storage Access Framework DocumentsProvider? For example with Turbo Editor you can enable "Use the Storage Access Framework" from the right hand drawer, and then it uses the SAF to open files, and can subsequently save them to the same location. Not quite as flexible as making files available via the sdcard, but perhaps better than nothing?