"In this case, I value:
- Protecting the guitar body for nicks and scratches;
- Low toxicity since I don't have a well-ventilated space to work on this;
- Ease of getting an even finish;
- Glossy, even surface is nice, but not essential in this case, particularly since my woods are pretty smooth;
- Needs to be compatible with a pore filler since I already used one;"
I can't think of any finish that does all of those things, and that's going to be the case pretty generally, no matter what your wish list is. As a friend of mine says; when there are a lot of different ways to do something it's a sign that either everything works, or nothing does. Finishes are an example of the latter.
To protect the wood from nicks and scratches you will need a fair thickness of tough and hard finish.Tough and hard are attributes that don't often go together: nitro is fairly hard as modern finishes go but not all that tough, in the sense that it scratches, and this is often the case with solvent based resins. Shellac is a solvent based resin that
is fairly tough, due to the fact that it's naturally cross linked, but it's not as hard as nitro. French polish is a method of applying shellac (or other such resins or mixtures) that gives the thinnest possible coating, which is why it has about the least effect on sound. You can brush shellac to get a thicker film that will wear better.Be aware, though, that shellac is soluble in alkaline water solutions as well as alcohol, and some folks seem to have alkaline sweat. There are some oil-resin varnishes that are both hard and tough; look for floor finishes, and avoid 'spar' varnishes: they have a lot of oil in them which makes them soft.
Note that any finish that has oil in it adds damping in proportion to the amount of oil. This tends to kill highs. Thicker finishes also can kill high end sound due to the added mass. Oils that soak in also add a lot of mass. Tru-Oil doesn't seem to soak in as much, and may form a better protective film than plain 'boiled linseed oil'. But oils don't cure hard, so they don't offer much protection against dings and scratches. They are easy to renew once the damage has happened.
This sort of dissection could go on all day, and leave you just as confused in the evening as you were in the morning. That's why I say that there is no 'good' finish.
The only person who can cut the knot is you. Dig out all the scraps of the wood you used to make the guitar and make up sample coupons. Get three or four different candidate finishes and try them out on the samples to see how they work. Choose the one that has features you need and drawbacks you can live with. If I were you I'd try Tru-Oil, shellac (Zinnser's is easy to get and usually pretty good), and one or two varnishes. Some luthiers have reported good results with Pratt & Lambert #38, now put out by Sherwin Williams, iirc. I've been using Murdoch's 'Ure-alkyd 500' floor varnish, from Sutherland-Welles. It's relatively environmentally friendly (the resin is made from the waste from a cheese factory, and they use citrus solvent rather than mineral spirits as a thinner), hard and tough, and it goes on thin. It can also be a bear to get it to harden on some tropical woods, ad with the hardness it takes some work to polish up.