The thing is, it isn't spontaneous breeding. For Cichlids, you can identify a pair forming and move them to a breeding tank. But in many ways, it then is no longer a breeding set up. It's their tank, that happens to be set up for breeding. Any tank that has paired up Cichlids in it is a breeding tank.
You can remove the pair and make it into something else, but as you've noticed, if conditions are good to the fish, why move to the suburbs when you can raise a family in town?
I had 15 young Parnanochromis brevirostris. I divided them into groups of 5, using 3 20 gallons.As they sexed out, I removed extra males. I got 11 males and 4 females. I added a 20 for the 4th pair,kicking some killies over into a smaller tank. The tanks didn't change. Same plants, filters etc. Their function did, as they are now brevi breeding tanks for pairs still too young. The extra males are in a community, and 3 rearing tanks are 4 breeding tanks. They'll be rearing tanks again of all goes well, and I'll need growout set ups, places for the pairs, etc. If I get a pile of fry some of the adult pairs will be rehomed.
Cichlid breeding tanks are always changing, always being adjusted to demand. That's why so many people breed dwarf Cichlids once, then cut and run. Success is expensive, in space, money and time.
A lot of people keep thomasi for years and get no spawning. You clearly have good water for them, and are taking the time to meet their needs.