The stack of tanks system does bring some issues. WC
is made much more difficult if gravity doesn't have the chance to be helpful. Assuming that all the tanks are running on the same basic top up water you can just use a return pump and bucket for filling. For the floor tanks you could also use a small maxi flow filter that can be modified to attach a water polisher attached to some aquarium line (urajoey also has a vid about water polishers i think). Using this as a replacement gravel vac would work as well.
Something i have not tried is to buy a cheap brake bleeding kit. One of the ones with a hand pump could possibly be used to do spot gravel vacs. My fish tend to deposit the bulk of their excrement in one quarter of the tank so it might work for me. I have never owned a brake bleeding kit so i may be wrong about the potential as a gravity free gravel vac. Maybe i'll buy one and experiment one day.
A third option would be to host a dual refugium on the top level. Return pump in each low tiered tank and a gravity fed return from the refugium.
Now if the water is all able to be filtered in one hit the options are limitless. MY LFS
keeps bettas in jars, but the jars have 2 air hoses. One drains a small amount of overflow water to a sump filter while the return line is situated above and just drips water from a holding tank at the top. I hate seeing the fish in small jars but at least they get their water filtered.
I think the trick with all things aquarium is to keep it simple so WC
is not a BIG chore.