Is it mandatory to use thermals in hand soldered PCBs?
In my opinion, if you are hand-soldering the board and have a decent soldering iron, you don't need thermals.

I regularly hand-solder on ground planes with stitched vias (RF boards) with no problems.  Because these are RF ground planes, they don't have thermal relief for the vias.

FWIW - my old Weller soldering station isn't happy soldering large ground plane areas.  On the other hand, my Metcal MX-500 with a sttc-113 tip works well.  I was doing a bunch of soldering of RG-58 shield braid on ground plane earlier today with absolutely no problems.