I'm a long time garmin user, just replaced my trusty 510 with a 530. I also use a FR935. My old protocol when riding with the 510 was to have it on and also record the FR935, then stop and not save the 510 at the end, but let the FR935 upload. On the occasional trainer rides where I wanted to upload the data from the 510 then I'd save and manually select the activity to upload.
But the 530 is driving me mad. Firstly it only uploads the next day, not on the day. If I go to activities - it shows the rides there, but clicking 'upload new' doesn't push the activity I've just finished to GC. But then next time I start the 530 (next day) then it suddenly uploads. Hoping this isn't a coding issue with me being in NZ and so +1 day date from USA/GC.
Second issue seems to be that you can't upload just a specific activity? I've spent way too long scrolling through the menus, but I admit I may have missed it as I'm still getting my head around the new structure.
Note that I'm connected by wifi - and this connection is fine, as evidenced by the way it uploads next day. It did the same with bluetooth via my phone FWIW.