May be worth his while to try a DIY piston soak.
Hyundai actually has a TSB for some of its engines to do this, so it’s not just snake oil.
It can be done without the special equipment.
Just pour an appropriate carbon dissolving solvent (Seafoam?) down the plug holes.
I’m going to sound crazy here. But I’d seafoam the absolute heck out of it. Low miles. Seafoam is an oil and carbon solvent. I’d do it. I have, actually, for other reasons and another make. Nothing ventured, nothing gained. Pour it in the crankcase, drive it a bit, see what happens. Ingest some in the throttle body. If it doesn’t work, you’ll be in the same shape you’re in now. You’re not going to damage the engine any worse than it is.