Shimming up a low nut is easy. I just used business cards a shim stock. Worked perfectly. Had to do it when the Graphtech nut I bought was too short.
Changing nuts is super easy - terrifying the first time you do it, routine after that. I did it out of necessity last year when I broke my nut and I kind of slapped myself for not doing it sooner. Just buy a few nuts, some good sandpaper, and small detailing/diamond files - you don’t need the expensive luthier nut files, though I imagine they are nice to have. But just going slow with a normal round and/or slot diamond file has always worked perfectly for me.
I recommend popping the nut off, filing it, and putting it back on to test. It’s slower but easier I think, and less nerve wracking. Plus it’s the only way to sand it.
Just take off a little material at a time.
To test, for each string, tighten it up a little until it holds the nut down, then fret the third fret right up against the second fret wire. The string should just barely visibly clear the first fret. Touching the first fret is too low, and more than just a tiny visible gap barely clearing the fret is too high.
For the types of sandpaper - I just used something like 250, then 400 and 1000 grit. But for that and popping the nut off I recommend finding some videos. It’s easy but will make much more sense if you watch someone do it. The tl;dr is you put a block against it and hammer it off, but watchign it will make it be not nearly as bad as it sounds. It’s easy and harmless.
For the nut itself I went with a Graphtech Tusq. It required significant sanding then shimming, but very little filing.
But I wouldn’t recommend this. Instead if I were you I would do what I discovered later - just order replacement plastic TRBX nuts from Yamaha. 304/504/604 are all the same, are inexpensive, and ship from yamaha24x7.com. And they have the angles just right.
The strings hold the nut down. So just a tiny amount of glue, very small drop of superglue maybe, should be more than enough to hold it in place when the strings are off. You only want to use a tiny amount so it is easy to pop off again; all it is there for is to keep the thing from falling off when you loosen the strings.