I've been trinity boxing since forever. I recently did a Mag'har team through dungeons. My take is that it's better for you now to get used to multi class setups. There is enormous benefit in group comp beyond tank and healer. Like debuffs, buffs, shroud and some really OP specs deal with mechanics better. Such as DH flight over some mechanics making them trivial.

I was DK + RSHM + 3 DEMO for most of Legion but now I'm in the midst of a change. The mobility requirements and interrupt requirements in BFA are high. So I am rotating out two warlocks for a rogue and a mage. Still a work in progress.

That will make my entire comp mixed. It'll be interesting to see how well I can grapple with all the rotations and gifts that each class brings. I'm hoping by maybe end of March I will be in M+ with them. Two melee on top of everything is going to be the most challenging part. For all this I think it's long overdue to have a keybinding for each slot to move independently. Moving them as a group is gone except for the rare occasion they need to move. Too much damage was being lost to moving them together or even partially together.