For anyone curious, here's what I ended up doing; my main problem was that the fresnel I had was in really bad shape, and using it made the image on the GG harder to see, and harder to focus on. I found a cheap replacement fresnel that appeared to be of the same thickness but was in great shape, so I went ahead and tried the following "sandwich": Lens -> (new) fresnel -> GG -> Eye.
The difference was striking... much brighter image, and much easier to focus... BUT how about focussing, did I mess up the focus plane by doing this? So I took a test shot to find out, trying to use a setup similar to what Doremus suggested. Here's the result; I was focussing on the "29" mark of the ruler. My test was probably not ideal (I only have a 135mm and a 90mm lens so I used the 135mm wide open at f4.7), but I am satisfied with the result. It looks like the focus on the resulting image is right where I wanted it, and even if it's not exactly perfect, it's well within what I consider acceptable for my use-case! In other words; Thanks everyone for all the info, I have a sandwich that I am happy with!
The only bad news is that while doing all this testing, I ended up breaking my ground glass. While it still holds in place and "works" there's a big crack right in the middle, so I have to find a new one
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