RPi & postmarketOS
This was extremely easy! As of 7/3/2023, I used their process to manually build the installation on postmarketOS's website. There is no real reason to duplicate that information here. I did use the plasma-mobile desktop in each case.
One thing of note that isn't completely clear on their wiki is that the pmbootstrap init command can be used to change your destination device/config. I did it first for the Pine Phone, then the RPi3, and finally the RPi4. I did not see a way to "store" different "sets" for different choices.
Another thing for folks new to Linux, be thoughtful about pmbootstrap install. I have my RPis all booting from USB; I used the --scard parameter. I also used sudo fdisk -l to make sure I'm choosing the right device (run command before and after inserting the USB drive). Oh, one more thing for newbies; if you're reusing a USB drive, you may need a new/empty partition table.
The first thing of note is that the touchscreen is flipped using the usual case.
The second thing of note is that it took a long time to actually boot the login screen.
The third thing of note is that after the first boot, I need to shut it down & reboot. If I put in the passcode, it would go to a blank screen for a while and then revert back to the passcode screen. After shutting it down and rebooting, I could then log in from the passcode screen.
Besides that the screen being upside down, what else do I have to say...? The mouse pointer is always there, centered. So two things that might need adjustment.
More importantly, though I got it connected to Wifi, I tried to launch Angelfish, the built-in browser. That just stayed on the splash screen for a long time. Nothing against postmarketOS; it works great so far on the Pine Phone I have.
Eventually, I will try the same thing on the RPi 4. I have to get it in the case with the touchscreen I bought. It might work better with better hardware.