2nd Update: all three bits arrived, I had ten days to try them and then make a decision. I used the following process to narrow them down, which worked pretty well for me.
First I rode in each bit on separate days, doing what I would have normally done during that ride (lesson, trail ride, schooling session etc.). I didn’t judge my horse’s reaction to the bit, just rode around and let him feel it out.
Next, I had three rides on three consecutive days where I directly compared one bit to another, including my own original bit. I warmed up in one bit and schooled for a couple minutes (no more than 5 - 10), then hopped off, let him have a quick break while I changed bits, put the new bit in, and schooled him for another couple minutes. There was a pretty noticeable change in him from bit to bit when I rode this way and I was able to easily rule out one bit each day. I would recommend having a friend on hand for these rides as a pair of eyes on the ground, cameraman, and extra hands to hold your horse in between bits.
I ended up with the Trans-Lozenge bit from Neue Schule. It’s an expensive bit, but made a big enough difference to be worth it, and so far my horse has only responded better each time I ride in it.
Definitely would recommend Florence from Try My Bits. She was very responsive to texts and calls, even on evenings and weekends, and spent a lot of time talking me through the different options.