I've looked up tantivy and quickwit. Quickwit uses tantivy as the engine. It has decoupled storage (awesome, only recently elastic announced something comparable) but is oriented towards log processing and esplitly warns against its use to power an user facing site search. Do you happen to know if there's anything like that with the same minimal footprint that can scale up and, importantly, down to serve the needs of highly variable traffic websites? Right now I'm looking at something with clustering capabilities and decoupled storage (e.g on s3) like quickwit

One of the reasons for not using Quickwit for user facing search is the latency: for example, you pay 70ms of latency when you make a request on AWS S3... and generally you expect latency below that figure. Decoupling compute and storage while keeping a very low latency may be then impossible unless ending up by caching all your data on disk :).

You can have a look at lnx (https://lnx.rs/) that is based on tantivy and is performing quite well. It's not yet distributed but the author Chillfish8 has some thoughts about how to do it.

Thank you! I'll look into it

There is also sphinx search which was open source before 3.0 version.

And it's open source continuation - Manticore Search [1]

[1] https://manticoresearch.com/

Do they support document access control like ES does?

Yes, Meilisearch supports ES-like document access control.