I got Custom Critters to maybe work again! It's only lightly tested, so use at own risk, share SMAPI log in case of crashes, and all that.
[I had to double-check, but it's open to modify and redistribute according to its
license!]
I could use knowing unofficial update policy on Cranber's Fireflies, too, if testing it turns out okay. It had the same light source reference ID format change issues in its json.
I'll try to reach out, I just know that mod authors are often busy! Meanwhile,
here's an unofficial Custom Critters update.
ADDITIONAL EDIT now that I've slept a little:
the github fork link, and I fixed the unofficial update naming format in the manifest.