I use Gadgetbridge[1] exactly for this reason. It's a FOSS app (on f-droid as well) that tricks wearables into thinking that your phone is actually communicating legitimately with the server.

Garmin support[2] is not really good. Some commenters here mentioned that Garmin watches can work without servers, maybe that's why.

It's not local-first by definition, because you still need a phone with an app, but it's the best I got for now (apart from finally assembling my PineTime watch).

[1] https://gadgetbridge.org/

[2] https://gadgetbridge.org/gadgets/wearables/garmin/