I don’t think it would be wise to keep the watch on for days or weeks at a time to be honest.

I also don’t see the logistics of how on arm charging would work. Would I just hold my arm in a fixed position for 30 minutes? Seems like a nightmare.

The Whoop band has an elegant solution. They provide you with a small (~30x30mm) power bank that you can slide onto the band to charge. The band stays on your wrist. When done, you just slide off the power bank and charge it again.