I spent a lot of time trying to find a good alternative and for what I was looking for, lightspeed seemed the best contender. I did not go to Lightspeed due to the platform fees and minimum commissions when requiring API access. If you don't require API access, that'd likely be a non-issue to you. If you do require API access, those minimum commissions and fees may or may not work for you. I'd recommend talking to Robert Morse regarding Lightspeed. I've spoken with him and he was incredibly helpful.
Seems threads asking for alternatives to IB are somewhat common, but there never seems to be a good response (or at least one that seems that it would work for me). It seems to be very case by case as to if the alternative really is better for that individual.
Good luck in your search and please let us know if you find a good alternative (and who that is).