Glock 21s Gen3 and earlier are great guns. They do need a bit of ergonomic/trigger related enhancements.
The slide releases need extending, mag releases need to be a touch larger and more elevated and the triggers - mine have a 3.5 pound pull.
Glock 21 Gen2(top) no finger grooves for me on the grip. Bottoms are Glock 17s, grip is a bit thinner, finger grooves ok.