Throughout Legion and BFA, I stuck with 5 tanks - 1 pally, 4 dks. This worked out really well due to all the healing going around. The downside is, the DPS isn't that great. I haven't attempted M+ Keys since 8.1 and only got up to 4, but doing melee with iwt works really well. It's also a lot of fun in PVP. The DKs swarm and stick to a target due to the IWT.

With Shadowlands and the removal of Hand of the Protector, I'm not sure how useful the Paladin will be for healing the DKs. Hand of the Protector is off the GCD. I've read that word of glory heals aren't that good, the stacks can't be used for a tanking ability if used for healing and it is on the GCD.