While looking for open source dive log applications on F-Droid (in preparation for my Advanced Open Water Certification) and I found an app called Subsurface and apparently it is as developed by none other than Linus Torvalds himself.
I also found this neat YouTube video of Dirk Hohndel talking about scuba diving with Linus.
You’re telling me I can self host a garmin server?
If you are asking whether you can use Garmin Connect without Garmin, I don’t think so. But you can use a different non-Garmin app with your watch, see https://gadgetbridge.org/ Obviously, it does not have the full functionality of Garmin, and even more obviously, you would lose the Garmin social functions without Garmin Connect (but there are open-source projects for that as well). (Also, with Garmin, maybe you will lose some of the functions anyway to the Garmin Connect+ subscription, right? :-) )