This seems like a fascinating problem domain! I'm incredibly curious what product you're working on - if you're taking this much attention to detail on recurring events, it seems like it's a step beyond most scheduling systems I've seen.

Heck, https://www.sciencedirect.com/science/article/pii/S147466701... and https://www.amazon.com/Declarative-Models-Concurrent-Cyclic-... seem to indicate this is still an area of active research. And, to your point, almost certainly too complex to try to encode into a Postgres index - that would be a paper-worthy project unto itself!

(I've had to implement the naive approach here, which nested-loops over dates and rules and finds ones that might conflict. Definitely not meant to scale, and would be a nightmare if not at a date-level granularity!)

No pressure to share, but would love to learn more!