I’m late to the party but I’ll still share. To answer your first question, yes this is how it happens.
My experience was similar, I think it also has to do with you unlocking upgrades for the guns along with the OCs, I’ve read someone’s suggestion for grey beards to try a Haz 4 or 5 without all of the upgrades and OCs to try and remember what it was like starting out. I need to try that at some point.
The other thing I’ll add is that I was just in that same comfort level and recently jumped to pretty much all Haz 5 unless I’m trying a new OC. Still dying a little more than I’d like so I wouldn’t say it’s cozy yet but Haz 4 started feeling too easy and I’m going for the challenge right now.