Termux is cool tech (including running full Linux in a chroot). I would also mention UserLAnd [0] to run full Linux distributions sand specific application with a simple install.
The thing I am missing the most is HDMI video out, as my retired Nexus 5 could do with MHL and a simple dongle on the USB micro port. That dongle could also provide power and another USB port for a mouse or keyboard. Very few modern phones seem to have any video out other than casting. I do see that DisplayLink does support Android, but I've yet to find a dongle that also provides USB C power delivery in a nice portable package.