Never mind, greedy me read the part where REU and GeoRAM do not conflict so I enabled both. GEOS hates having REU and GeoRAM enabled at the same time, so I disabled the REU and now it boots up.
I toyed around with it a bit and it seems to be fine, no video instability or graphical glitches that I could see in both 40 column and 80 column modes.
Please excuse the horrible photo, my phone camera hates LCD screens for some reason. On my screen, the background is grey, not blue like on the photo. I am using HDMI, if that helps.
I just noticed that I have GeoRAM set to 4MB and REU set to 16MB on my C64 core and GEOS loads fine there. Not sure if GEOS hanging with both enabled on the C128 core is due to differences in GEOS128 itself or some bug in the GeoRAM/REU implementation on the C128 core, but I would not consider this issue something to hold up a release for.
Thanks erik5! This is honestly awesome work.