I run the built-in automatic rpm-ostree upgrade service every 6 hours.

If you think that's too inefficient, maybe read the docs for shutdown.target and see if you can use that to run an upgrade service before shutdown?

I'm not too experienced with that part of systemd but it seems like it could be a "proper" way to run things on shutdown?

I use uBlue and update manually (using a custom alias/script) whenever I get the time, like say during my lunch break or something. Reason being, I actually like watching the update process and seeing what gets updated, watching out for major version number changes or major package upgrades, and if I'm interested I may look up some of their changelogs to find out about their new features etc.

and being forced to reboot

You should be forced to reboot though? And if you don't want to reboot, can't you just do an --apply-live? I mean you'd still need to reboot for a kernel update but for the most part, you should be able to use most of your new packages without a reboot. And this holds true even more so if you're updating Flatpak/container/Nix/pip/cargo/brew packages. And I hope you're not doing the rookie mistake of actually installing stuff at the ostree layer instead of using Flatpaks/containers/Nix etc.

You could change the action of the power button I guess, some dconf stuff?

#!/bin/bash
sudo waydroid upgrade
distrobox upgrade --all
flatpak update -y
flatpak cleanup --unused -y
rpm-ostree update
shutdown -h now

On KDE this will work too. I simply alias that command to "update" and "upfin" (including the shutdown) but yeah that needs an improvement.