OK thanks for pointing me in the right direction! I got it working pretty much exactly how I wanted it to.
We may do more than one action per turn with certain upgrades in our game, so I wanted to avoid using Turn End schematics. I tried a default Ability End battle animation, but it was causing every ability to do nothing, unless I put a Calculate Action node as well. Maybe it would still work, but I thought it would get messy once we started adding more unique battle animations to abilities individually.
Then I found the Battles > Battle System > Phase > System Action Schematics > Finish Performing Schematic setting, and I put the schematic that removes the status effect there. It works perfectly as far as I can tell. Do let me know if this is a bad idea for some reason, but from the tooltip it seems like it'll work just fine.
I set the schematic to remove just one stack, so we can have status effects last for more actions by just adding more stacks. Unfortunately, there's no way to remove just one stack of a certain Status Effect Type, so I'll have to go through every status of this type one at a time. Not terrible, but we are planning on making this a big part of our battle system so we may have a lot.
Neither of these are super important because I know ways to work around them, but if I could make two small requests: One would be what I described above, being able to remove one stack of every status effect of a certain type from a combatant, and Two would be the option to add a Secondary Status Effect Type to status effects, similar to how you can add a Secondary Ability Type to abilities. Again, not super important, but if you think they are useful ideas, we'd certainly appreciate it.
Thanks for your help with my issue, hopefully my solution is fine to use, and hopefully if anyone else is trying to do one action duration status effects, they'll find this thread.
EDIT: OK after some more testing I found the Finish Performing Schematic also triggers when a combatant dies. I guess things like that are considered a System Action. However, I was able to just filter out actions like that from within the schematic itself by using an "Is Battle Action" node and setting the Action Type to Ability (we don't use base attacks in our game). Now it truly seems to be working perfectly, but again let me know if I'm still missing something critical.