Well I don't recommend the 822 from Verizon unlocked albeit its a good upgrade and cheap, like you could get it on eBay new unlocked for 100-120$ but its had issues with mms or SMS on TMo. 810... I wouldn't recommend, TMo did EOL it before it could get black firmware, so off the bat no choice but to use preview for developers to get more updates other than just Lumia amber firmware.
I'm on the 521 as well, but I'm shooting for Lumia 1520.3 as my next upgrade. Check newegg or b&h store online, 519$ and 489$ respectively.