The rootfs on external storage guide is well-written and works every time for me. You do need to be able to edit files with vi and run a few basic commands, but otherwise it’s very straightforward and allows a painless transition to moving your entire system to run on external storage/SD. Once it’s set up it will automatically switch to using your external storage so there are no additional steps beyond the original setup. The internal flash is mounted on /rom and everything you do going forward is as if you booted directly from the external storage, with the exception of firmware upgrades.
-Larry