Generally, Thrawn is the better lead until either Palpatine gets his zeta or you have some Sith you want to throw in the mix.
Tarkin's leadership ability isn't great, and Empire is flush with much better leaders. Also, his expose is a zeta ability, and by zeta standards, it's really bad. Like, when folks start talking about worst zeta in the game, Tarkin's leadership ability is a contender. Palpatine's health leach can trigger that defense down while it's still useful, but if you have Palpatine on a team with Tarkin, then Tarkin should not be the leader. He actually might be the worst Empire leadership ability, which is less a mark against Tarkin's lead, and more a sign of how good a lot of Empire leadership abilities are.
Vader lead, meanwhile, is very good, but not really meant for arena. It's made for much more drawn out fights, and Arena is fast-paced. Use Vader's leadership ability and especially his zeta in raids.
Now, as to your lineup? The main one I question is Royal Guard. Honestly not a great tank, and has a bad habit of not taunting until someone's one or two AoEs away from dead, or debuffed into ineptitude, or someone important is fractured. I'd much sooner advice Stormtrooper, who while not amazing, is pretty good on almost any Empire team. Shoretrooper is the best Empire tank, but you can't get them yet. Alternately, you can run Empire with no tank. TIE Fighter Pilot and their AoE buff block are much better than you might think, and you'd be improving a character for your fleet. Mod for crit chance and Snowtrooper can spam AoEs and help you gain a lot of momentum once enemies start going down.
Or slot in General Kenobi. He's a great tank on any team.
Or, if you run Palpatine lead, Sion is a perfectly viable tank, even at three stars.
Sith Trooper's another tank that can be fantastic for this team, but you won't be able to farm them any time soon. I'm guessing you got yours from a pack or something?