Final edits of a paper at the moment, aim to submit next week.
Perturbers are easy to add, though a little poorly documented at the moment. Additional physics right now are J2 of jupiter/sun/earth, and GR corrections for the sun and jupiter.
Biggest speed gain is that I have a custom SPICE reader that is multi-core friendly (I re-implemented a lot of the SPICE standard in rust), and it is used as the source for planet positions. Being able to skip planet integration leads to massive speedups.
Anything published on your integrator and its modifications?
One nice feature of ASSIST (from what I remember, its been a while) was that I could add in more perturbers and crank up the gravitational harmonics if I wanted to. It sounds like you support that too at least for perturbers?