You are SO overthinking this! 
Classes become important at Mythic raid level, and M+ from 10 on up, definitely at +15 and onwards, but for Normal and mostly Heroic raids, and lower M+ keys, knowledge and practice is much more important.
Rogues are Flavour of the Month. Warriors are well in, in both tanking and DPS. Locks a bit less so for M+, but still entirely viable unless you’re pushing, and quite strong in raids.
Class is much less important.
Step 1: Look up dungeons and raid guides on YouTube. Run them on LFD and LFR if you’re not already familiar with them, to understanda the layouts and mechanics.
Step 2: Get the Usual Suspects of addons: DBM, Method Dungeon Tools, Angry Keystones, GTFO and whatever else, if you don’t already have them.
Join Fail Train Discord
and Scared of Dungeons on your Alliance
and Calm Keystones on your Horde
These are cross realm organisations that aim to provide friendly activity for people. They’re not going to tailor you a personalised course, or anything, but sooner or later they will provide you an opportunity to join groups that will not press you too hard.