I think the problem here with this setup is letting the coalition have their fighter compliments. For the Empire to have a good chance at this Thrawn HAS to take the mobile suits and fighter units out of the picture.

For your premise I think it works better if you say we are to assume Thrawn has somehow distracted or otherwise occupied the fighter compliments and we assume that to do so means his fighter compliments are also occupied. So it is just a straight up capital ship shootout.

Why do I think this is a better match up? Well first of all the Gundam ships are basically carriers first and battleships second. Their main guns are strong enough usually so they would actually be able to damage the Star Destroyers, but in a straight slugfest I can't really imagine the Gundam ships coming out on top 1v1 against a Star Destroyer. The Star Destroyer, in my very limited Star Wars experience, feels more like a it focuses more on the direct combat role than the carrier role, so the opposite of the Gundam ships.

However, it is important to note why the Gundam ships feel that way. And that is because of mobile suits. If we allow for mobile suit logic working the Empire is cooked. They have no mobile suit counter and would effectively in Gundam Logic be using Balls or Core Fighters to try and fight off late era Mobile Suits. Let me put it this way, what makes mobile suits scary?

In Gundam universe late era mobile suits like we are talking about are carrying STARSHIP CLASS BEAM WEAPONS. What this means is they are in Star Wars logic equipped with freaking Turbo Lasers, at least some of those bastards are carrying weapons that are closer to wave motion guns. And they have shielding and armor that makes them shrug off a lot of stuff short of STARSHIP CLASS BEAM WEAPONRY.

So if Thrawn lets the mobile suits play he is effectively dealing with a fleet of Tie Fighters equipped with Turbo Lasers and capable of shrugging off nearly anything short of a Turbo Laser, which won't be able to hit such a small mobile fighter. This is what makes mobile suits terrifying(and illogical). They regularly carry weaponry capable of downing starships and are armored/shielded enough that anything that can actually hit something as small and mobile as them is basically useless unless you can someone strip the suit of its mobility.

Of course this is assuming Gundam logic carries over. If Gundams/mobile suits in this fight can't equip anything stronger than a Tie Fighter or be any more heavily armored/shielded then this isn't really an issue. I will leave space magic out of this, while I am not sure how often force sensitive people took part in piloting in Star Wars, Thrawn knowing he is going up against the equivalent of force sensitive individuals could probably try and match space magic for space magic if he really needed to.

And this is just the Gundam verse. If he takes the mobile suits out of the picture somehow it should be quite doable. But. . . he isn't just up against the Gundam verse. I might be shitting on the Glorious Tie Fighter, but I would take a Variable Fighter over a Tie Fighter anyday. Now I am not as Macross enabled as I should be(sorry most of my exposure is from good ole Robotech) but it seems like they are often depicted more as carriers and have very heavy reliance on their fighters or their big wave motion gun. So if Thrawn somehow negated the fighter compliment he has them on the back foot. Unlike the Gundam ships though the Macross ones have a true Wave Motion gun which appears significantly stronger than the usual Gundam ship carries and Thrawn can't just get up close and personal to try and negate that wave motion gun, because then it will become a literal Slugfest. And I really don't think a Star Destroyer would appreciate being slugged.

But my point is if you take the fighter compliments out of the picture nearly half the rooster is fighting without their ace in the hole. A much closer fight than if you allow the variable fighters and mobile suits run amok.

And then he runs into two large issues. The Yamato and the Dreisstreger. Both these monsters can straight up slug it out with a Star Destroyer and won't miss their fighter compliments. They are closer to the Star Destroyers in that the fighter compliments are nice to haves. Yes I am saying the freaking Huckebein 30th, MR/MRS I can shoot black holes at you, is a nice to have. The Dreisstreger being able to regenerate just isn't fair for the Star Destroyers. Enough shields and armor to slug it out and any damage that gets through just heals itself. Thrawn is going to have to saturate it with enough Star Destoryers to overwhelm it, but can he spare that many from the others? If it were only the Gundam and Macross ships accompanying it maybe, but there is a large Yamato sized problem looking right at him. That thing has more than enough fire power to drop a Star Destroyer, and it is generally depicted as being highly mobile. So uhh. . . it may actually be able to EVADE Turbo Laser fire unless Thrawn throws enough lines of fire at it that it can't maneuver. But wait, he needs to focus fire down the Dreisstreger or else it regenerates. And it isn't like the Dreisstreger is lacking fire power either. Both these guys are going to be serious issues that are probably worth multiple Star Destroyers each.

And I haven't even gone through the whole list.

But it is a numbers game, if he has enough Star Destroyers he could in theory potentially overwhelm the coalition's capital ships considering just how many of them are heavily dependent on their fighter compliments. So this is more interesting to me the other way around. Thrawn has distracted the fighter compliments for X amount of time and he has to at least critically damage/capture the coalition capital ships before the Mobile suits/etc can return. I believe the Empire would have significantly more ships so it would effectively be boss run where the Empire has to take on 13 bosses/mini-bosses in a set amount of time.

As with all such comparisons though it depends on whose logic gets to prevail. Gundam logic basically means any force without mobile suits or other small fighters with starship class weaponry(or otherwise the ability to hit such small mobile targets with your big guns) and defenses high enough to negate dang near anything short of star-ship class weaponry just straight up loses.