Firstoff, NM cable cannot go outside. Fullstop. Doesn't matter if it's in conduit: outdoor conduit is not "indoors". It actually tends to fill with water due to condensation.
If you really, really, really need to use cable outside, UF-B or MH feeder are the right stuff. However UF-B is very flat and wide, and does not work inside conduit. (the conduit must be outlandishly large: 2" conduit for 6/3 UF-B).
Given the distance, size of the wires and the fact that you're going to a subpanel, you might consider running aluminum wire to cut down on the cost. There is nothing wrong with aluminum wire at these large sizes. Further, at #4 and larger, you can use the same THHN wire, no need for white neutral or green ground.
You can decide for yourself whether to run conduit all the way across the crawlspace. It's not that hard once you get the hang of it. For novices, I strongly recommend EMT conduit because it has an "Undo" and a "Save game" button: you can do a few segments, take a break, snap photos, get feedback, undo mistakes, and resume as needed. Then the hard part is behind you; actually pulling the wire is just a "victory lap".
(in conduit you always build out the entire conduit run before you pull ANY wire. Never assemble conduit over wires: that's simply wrong.)
Some people say "oh, PVC is better"... I suppose that's true if you don't glue it, but then you better really anchor it down well, or come back when it's finished and glue it. Also EMT provides the ground, so one less wire.
By the way, if your garage is an outbuilding, you can't run separate circuits, you must do a subpanel. On this forum we strongly recommend people "Think BIG" when choosing a subpanel. I would choose a 24-30 space even if it is laughably bigger than your main panel. Spaces are dirt cheap, and when you add something to the house, it's nice to find the spaces you need waiting for you, instead of having to do a bunch of expensive rework. While the heaters will largely "max out" the panel, if you aren't running the heaters you could run a machine shop, compressor, welder, even an electric vehicle charger. These days an EV charger is something homebuyers will pay extra for, and you'll be 90% there.
One other reason to go THHN over cable: With #6 copper or #4 aluminum,
- NM and UF cable are limited to 55A (breaker at 60A).
- THHN can go to 65A (breaker at 70A). So in that case you could run a 120V saw + dust collector even with both heaters going full-bore.
Your 4000W heaters need to have 125% of that provisioned for them, so 5000W each or 10,000 watts the both. That's 41.7 amps. That leaves you 13.3 or 23.3 amps to play with depending.