I'd like to add an AFCI to the middle circuit here. But that circuit shares a tandem breaker with the circuit above it, so I don't know if there are any subtleties of this setup that I don't understand. They don't appear to be a multi-wire branch circuit--they have separate neutrals going to the neutral bar. Can I just move the middle circuit to a new slot?
You have a panel with 12 spaces and you are using 4. Just move the red wire to an AFCI in a new space in the panel.
I would also move the lower black wire to where the red wire is now, so it becomes visually obvious that both wires are meant to be on the same pole. I'm all for using a rainbow of colors in conduit/THWN, but in residential use with NM cable, red always means something special in some way.
On a tandem (I call them double-stuff) breaker, both circuits are on the same pole. It is very easy for someone to confuse them for a 2-pole breaker, which MWBC's require for safety. So when an electrician sees a panel with a double-stuff with red and black wires, he'll pause and follow the wires just to make sure that mistake wasn't made.