Would Elive be a good choice for the Pi 400 I just ordered?
Unfortunately, Elive does not support the CPU architecture that teh Raspberry Pi uses. We're hoping to fix that eventually (in the long term); you can see @triantares progress for an ARM-based version (that would in theory support the Pi) in https://forum.elivelinux.org/t/pinebook-pro-review-will-progress/2287 (if you have permission for that category). (If you don't, just read a few messages for a little while, and you should get it automatically.)
You can join the discussion here: RaspberryPi* (ARM-based) version of Elive, HOW?.
The pinebookpro and the pi share the same architecture but have a very different way to boot up.
It should be feasible to put Debian 10 (Buster) on the RPi and shoveling Elive into that.