Generally, I am the guy who says play what is fun. The issue I have right now, is that I feel like playing a few different classes. First
Returning: From the start of Shadowlands - Its been suggested I may want to start over just to get used to some of the big changes that have occurred since.
My play-style is solo. I have a group of friends I will be playing with, but 80% of the time I am on my own due to work hours.
Classes I most enjoy but don’t really know the state they are in. Specifically there ability to solo.
Prot Warrior: Can they still solo pretty much all questing content?
Hunter Survival and maybe MM: I don’t enjoy BM overly much. But how is survival? MM… I may wait as it seems to be getting ready for a ton of changes.
Death Knight(Frost): Frost is potentially one of my favorite all time specs. But I did have some issues on elite quests that I didn’t have on other classes back in Shadowlands. Is that still a thing?
Welcome back!
But you’re all set. You don’t need advice from us. Just pick one - or use all three.
There really haven’t been big changes since BfA./Shadowlands, but you can start again if you like.
ALL tanks, including prot Warrior, can easily solo pretty much all questing content.
Did you ever consider swapping from Frost spec to Blood for the Rere-Elites?
The one thing you should consider is Delves - think of them as solo mini-dungeons. If you’re solo a lot, they will be your main gearing ladder. Search for them on YouTube to get a feel for what they are like. Work through trash, through a lesser boss, toa final boss.
We are given Brann as a companion during Delves, and in the upcoming season you will be able to spec him as any of Tank, DPS, Healer. He wasn’t a Tank before, so melee specs like Frost DK might have had a harder time then they should have with the bosses. But maybe with Brann tanking, that will improve. We don’t know yet. (But I still think the best solution is to change spec to Blood to solo big Elites 
)
Frost DK is in a great and very fun spot right now, so if you liked frost back then I would suggest you try it now