i have successfully managed to join smartthings v2 and smartthings v3 on one account /email address, so when you log in you can see the 2 hubs and all the devices that is shared on each hub and all of them combined. finally i can stress less lol. z-wave products are far more successful than stuffing around with sonoff and tasmota, as you can see ive tried many many products i love sonoff with tasmota flash but the delay time on the manual switch has always been a downside, so i managed to get some cheap fibaro switches and connect them on the v3 hub and now when i log on the smartthings app either classic or the new one i can see all of my switches and interact with them all. if anyone needs images or a walk through shoot me a message and ill be glad to respond