In general I feel it's easier to maintain fewer repos. Kotest extensions are a bit of a pain to keep up-to-date now, but on the other hand they don't actually need to change that often, so maybe it's a sign that we would've spent effort for little gain by keeping them in the main repo?
One concern I had was seeing compatibility changes across framework/assertions/property, but as you say we don't really do things that have big impact across those boundaries.
Regarding releases, we could just do them as fast as we'd like even with everything in same repo. It's like when Kotlin does a release, sometimes there's not really anything new that affects us, but other times there are. I guess it's currently the same users who only use assertion, or only property-testing, etc..