Well, this could go a lot of ways, depending on where in Ra's old holdings Thrawn finds himself, or where in the Pegasus he finds himself.
Pretty sure the Empire of the Hand had a way bigger fleet than what Thrawn took with him back to the main part of the galaxy later on.
If this is Legends only, then he likely had effectively his own fleets of stuff the size of dreadnaughts to gunboats, with the industrial base to build his own frigate-cruiser fleet with some heavier bits, but only one ISD and that was the
Admonitor, plus one Victory 2, IIRC which he left there with his trusted officers.
Thrawn had rescued many species from slavers and the like in the Unknown Regions adjacent to Chiss territory and between Chiss territory and the area controlled by the Pentastar Alignment/Imperial Remnant.
Per Wookiepedia, this is what the Empire of the Hand had:
"The Hand held
Imperial-class Star Destroyers,
Victory-class Star Destroyers,
[20] Immobilizer 418 cruisers[21] and many more.
TIE/LN starfighters,
TIE/sa bombers,
TIE/IN interceptors, and
TIE/sr starfighters[10] were used as well.
Zeta-class long-range shuttles,
[10] Lambda-class T-4a shuttles served as primary
transports for the Hand's forces.
Strike-class medium cruisers, and
Carrack-class light cruisers were used as well.
Golan space defense platforms also made its use with the Hand.
[6] All Terrain Armored Transports,
All Terrain Scout Transports,
Imperial Juggernauts,
Swoops,
[2] and
Imperial Troop Carriers were used by the Hand for support of its ground forces.[16]"
So Thrawn has access to limited amount of ISD-1s, VSDs, and a lot of smaller warships, including notably Strike Cruisers and Interdictors. Not a bad mix, and depending on numbers, he might be able to sacrifice a few ships to create some sort of mobile refinery and shipyard they could build shit at in empty systems.
However Thrawn has never dealt with transporter/ring/beaming systems, and Asgard-type GO AWAY 1-way beaming is something he has no immediate counter to besides running and hiding.
And while Jaffa/Wraith are usually crap in modern combat, TIE's are likely fairly evenly matched against Death-gliders or Darts, and a few squads of Jaffa/Wraith beaming onto the bridge, or just a naquaduah bomb beamed onto the bridge, would mean he was personally vunerable to a decapitation strike at any given time, and so were his entire command chain.
Thrawn also likely hasn't seen a sensor system that works in hyperspace and real-space in the same fashion, nor sensors with 10s of lightyear of range from individual nodes.
If Thrawn can survive the first few encounters and capture enough tech/captives, he should be able to get a decent enough understanding of the new foes/new physics he's going to need to adapt too/utilize to carve out a fiefdom for himself somewhere.
Abydos actually would likely suit his needs fine, and qualify for the criteria, since this is after Anubis blew it up, no one would likely bother to go near the place except to salvage battle debris, if no had already done so. Baal may actually ally with Thrawn, if they meet on non-hostile terms, because Baal is cagey enough to realize Thrawn has no predisposition against Gouald and is using unknown tech.
Now if it's fucking Quetesh who finds Thrawn, then a lot of ways it could go; the way Vala described her, Quetesh was not the worst snake, but probably would have double-crossed Thrawn when she felt she had an opening.
In Pegasus it would be way more dicey, because a Wraith hive-ship, or even cruiser, might be able to tank and regrow from turbolaser fire faster than Thrawn could put one down, depending one what fleet he has with him.
And once the Wraith get any of Thrawn's people for food, they won't stop throwing ships at Thrawn till he's dead and any nifty bits are in the hands of Wraith scientists.
If Thrawn runs into the Asurans, or Pegasus Asgard, then it's a whole other ballgame. If he meets the Pegasus Asgard, he may actually agree with their plan to detonate the Pegasus stargate system, simply because they are such vunerable two-way choke-points that would have to be constantly monitored and reinforced if he ever wants to hold territory.