What photos do I need to get the ID further?

between Bourletiella and Deuterosminthurus you need an observation of the courtship ritual.
In Deuterosminthurus : male and female head to head dancing, initiative comes from the male. When dance is correct and female is in the mood, the male will deposit a spermatophore in front of her and he will invite the female to walk over it and to take it up.
In Bourletiella : male and female walking after each other (male first) and besides eachother, initiative comes from the female : she nibbles on special dorsal setae of the abd.6 of the male to stimulate him to deposit a spermatophore that she then will take up.

That's not going to be the easiest thing to find. But yet, today I did find two globulars headbutting each other before one chased the other away. However, one looked like Deuterosminthurus bicinctus (two spots), and the other looked like the one in the photo we're talking in the comments of right now (no spots). Could that have been mating?

the 'headbanging' that will be Deuterosminthurus. And in D. bicinctus there is a form without dark spots : forma flavus. This form is not yet reported from the nearctics. So it could be scoop! But you need to see the courtship ritual. The fact that it got chased away could mean it was a mismatch between different species ;-)
Awaiting your courtship pics eagerly :-)

I hope they're useful! I only took 3, I didn't realize that I was seeing something interesting at the time.