Just play retail, the people complaining are sweaty tryhards who are facing issues you wont even encounter.
Just coming back you'll have a long time before you are a top rated pvper, in a mythic guild full clearing, or in a group pushing the highest keys. I'm not saying you are bad but its just not realistic that you will be facing any of the slow ques being brought up for a long time. I would say you should probably play horde though, horde population heavily outweighs alliance overall and I think thats why these two are having issues.
Shadowlands major issue is a lack of content for people that have been playing consistently since launch, but if you took a break there should be enough there to keep you busy for a while.
Or just play both, classic tbc will get to the point you are just raid logging and then you can hop on retail and mess around. They are both included in the monthly sub so why not?
also if classic wow is too grindy for you, don't play ashes. Its going to be way more of a grind than wow or new world. Its more of a spiritual successor to everquest than a clone of wow or new world.