Acro not holding angle

I am new to APM, coming from OpenPilot where I am used to having Rate mode configured so that, with enough I term, the copter will hold its angle, with no stick input.

So far, in my attempts to tune Acro with the same quad, I cannot get this to happen. I can add Rate I, but it never really locks in, and instead slowly returns to level, and if I keep raising it, I get oscillations.

This is not an autotune question, just a manual tuning one. Is this supposed to work? This is currently on 3.1rc5, and a basic 500-sized quad. Thanks in advance!

[edit] just found ACRO_TRAINER set at 2 (Leveling and Limited). Will try at 0. Assuming that makes a differnce, it is truly supposed to (theoretically) hold that angle indefinitely?

Yes, that is correct. With Trainer set to 2, it will self level. With it set to 0, it will hold the angle tenaciously as long as you have your PIDS setup right.