I'm suprised there isn't a self-hosted solution for this yet. Maybe there is... I've never looked into it.
I have looked around a little but the DIY community around those devices is not nearly as mature as one would hope. It is slowly getting there but it will probably be 5-10 years until there is something that can do everything a "commercial" product can do today unless someone gets really pissed off and decides its their life goal to make something like that.
I am pissed off enough but I don't have the needed background in the medical/hardware field and even if I spend the time to learn and build it there are projects out there that are already better than anything I would probably make (maybe?). I don't know. Life is just too damn busy for me at the moment to try to work on something like this.
You might look into Oura Ring 4. I don't have one, but it's a lot smaller than a watch. Supposedly you don't have to take it off in the shower.
I've looked into Oura but I really don't like rings. Its a personal preference. I would rather have a watch than a ring.