I haven't played since season 7, the game progression has become totally different.

So the situation is like this:
I want to get my first mythic by using the season ranking resplendent sparks.
Back in season 7, I was struggling hard with dance of knives rogue at T3, but the moment I switched to blood wave necro, I was 1 shotting bosses using starter gear at T4 (highest back then). This made it clear that choosing the "wrong" class makes progression so much more difficult.

That was back then, this is now. Is it still true that starting the "wrong" class handicaps progression? Put it another way, if after investing dozens of hours in one class and it sucks, then starting another class and within a few hours it's already wrecking the previous class?