If you're concerned with getting used to a tank role, keep in mind that whichever you pick you can just queue for lower leveled duties until you get your feet under you.
At the end of the day though, if you already have GNB and are enjoying it, that's all you need~

That said, if you still wanna hear about the othere:

WAR is probably the best one to start with.
It's got the lowest actions per minute so you can spend more time focusing on what you're doing and less on your rotation, and is pretty "Oops proof" with its survivability.
It's also the most flexible as it's buttons aren't necessarily all about pushing them when they come off cooldown.

DRK is pretty streamlined because at its core it is a 1-2-3 combo and a bunch of oGCDs that build around that. Sometimes it can get a bit hectic, and occasionally you might have trouble fitting a tank skill in during oGCD packed burst windows, but it a very flexible tank and that will only be a part of the learning curve I wanted you to be aware of. It's a solid option for a first time tanker. I started tanking with DRK in Stormblood.

Paladin can be a solid option coming from Samurai, as it has a pretty defined rotation that you may take to quickly. And like SAM it has a butt load of buttons. Most of them are options for this situation or that, so it's kit is pretty well rounded and once you get the hang of it, it is very good at supporting a team.