S6 is way more complex and rich. Nitro or runit would be simpler alternatives; maybe even https://github.com/krallin/tini.

Thanks! Reading some of your other comments, it seems like runit or nitro may not have been a good choice for my usecase? (I'm using dependencies between services so there is a specific order enforced & also logging for 3 different services as well).

You seem to know quite a bit about init systems - for containers in particular do you have some heuristics on which init system would work best for specific usecases?