It sounds like the difficulty for source-based distributions comes from trying to do an in-place upgrade that makes incompatible changes to the ABI. So changing to an entirely different distribution would be at least as disruptive (though possibly less time consuming) as doing a clean install of Gentoo using a new ABI.
Starting a few years ago I partition all my drives with two root partitions. Once is used and the other is blank. For precisely this reason. Sometimes it's easier to just roll out a brand new stage3 onto the unused partition, build an entirely new root, and then just move over to that once it's finished.
The bonus is you can build your new system from your old system using just chroot. Very convenient.
Several OSes I use on a daily basis (FreeBSD, Fedora CoreOS, and Fedora Kinoite) adopt related strategies as part of their regular (binary) update processes:
