"recurring upkeep like that you mostly just want to see at a glance how long it's been" EXACTLY
I think as a 'future-proofed-concept' probably letting the user have the choice of whether the custom activity is displayed is probably best.
I know me personally, if there are columns that I don't plan on using, and I can't remove those from view, it drives me crazy, though some would argue it's a short drive, lol.
So to give you the cut and dried answer, let us choose if the column/activity is displayed.
To add on to this thought, I got to playing around with the status indicators today. I saw where I can set up a color-coded dot/symbol if it has been X days since 'being cleaned'. Piggy-backing on this function, I'd love if there was a way to also use those custom activities in the status indicator section as well. Ex. If it has been X days (10) since watering the plants, show a (blue) dot next to that animal.
I'm not sure what the level of effort for that is if it is a custom field versus a system field... I'm not sure if y'all would be willing to add any new activity types, I know y'all already have so so many, but it's honestly great. But maybe even adding two types specifically for Live Plant care/maintenance and one for Misting Machine refill/maintenance? I know I am not the only one out here attempting to keep track of that.