"/var" is almot full, with a 84% used, and I want to try to decrease that percentage. I've removed logs and all I could remove, but that percentage is still pretty high.
Searching around, I found that apps installed through Flatpack are stored in "/var/lib". This may be my problem? Is there any way to store Flatpack apps on "/home"
There probably is a way to change the storage location, if not, or if you want to only move a few (perhaps the devices have different performance) you can merely move the directories and symlink them back
Typically I have just copied the tree and symlinked it. Not tried that with flatpacks though. There are also ways to remount folders too if symlinking does not work.
Generally too I typically do not break stuff up like you have for this reason. Instead I go with separate home partition for easy upgrades and lump the rest into /. If I want to break stuff out I use LVM which is more flexible.