Pubs are definitely good practise for getting the muscle memory.
Obviously 6s is good 6s practise, but the issue is you get the opportunity to go for shots and angles way less.
Practising in 6s (scrims) to me is more about thinking if sniping right then is the right decision/if the way I position is correct. The right opportunities will come up more as you learn to make the right calls. I basically ask myself "should I snipe right now or a different offclass/scout?" every time I want to switch to it. Sometimes I decide to do it anyway against my judgement and it stills works out. So it's more about learning than kicking yourself when it doesn't go right. Also in pugs/mixes nothing matters so snipe your heart out lmao
Aim trainers aren't as good as people claim, but they're better than nothing if pubs aren't your thing. Avoid if you have better options (aka most options)
HL isn't as good as pubs imo (if your intention is to get better at 6s) because you're always thinking about spies, getting spammed w detonator, perma counter sniping, etc. In pubs, like 6s, you don't have to think about these things as much (because the players are worse. not because they're not there) and you can just chill and find out which angles work, but without the pressure of having to actually hit the shot