Also, one big advantage of IB is they allow access to the SPX options overnight and curb session, if that is helpful to you at all. They are the only retail broker that I know of that offers full access to this session. Fidelity offers access to part of this session, a couple of hours before the regular open.
Fills on vol through IBKR are the best I’ve seen at retail.I’ve worked identical orders with IBKR and with other BDs and IBKR has always come out on top. I don’t know if it’s the MaxFill algo but I generally not adding liq.