I built a trig as a flat bar with Shimano flat mount brakes (UR3000 I think) and 700×40 tyres, could go bigger at the front not much bigger at the rear, I heard 45 could fit but I’m not sure at the rear.
I already had the wheels but it did have the benefit of raising the BB a bit so less rock strikes off road.
I ran it as both 2x grx 400 with 46/30 and 11-34 cassette with tiagra flat bar shifters and 1x with 40t grx chainring (it’s the same chainset for both) and and 11-46 with the 5120 mech
I did size up on the frame to go flat bar but was in between medium and large anyway.
I use my rigid MTB mainly on road and gravel and have slaughters front and rear. Nice and quick for that but wouldn’t feel confident in them on anything more technical
Someone mentioned to me that as its a longish bike the front might be twitchy with a carbon fork uphill, looks like i can ignore that and get cracking with ordering a fork and some decent rolling tyres.
I would, you are taking about 1 service probably. For the sake of maybe £50 extra (if a minor service next) it should be easier to sell even if it doesnt increase the value