If you don't have a program to tweak the image yourself, you can try a brute-force method of bringing out text from the few areas which have too much contrast: get a yellow transparent report cover and hold it over your computer screen.
In addition to the previous suggestions, with image software you could try enhancing your image by loading a copy of your image and tweaking the color balance (I use IrfanView on Windows). This may not be effective with such a challenging image, but it sometimes works for gravestone photos where switching the image to negative isn't enough.
Another thing you could try is to look for published transcriptions of the church book. If someone happened to transcribe the original by looking at the actual book instead of a photo, they may have been able to read text which was not picked up well by the camera when filming. This is another technique which perhaps is more help with gravestone photos, but you can't know a transcription doesn't exist if you don't look for one.
If you haven't already, double-check to make sure that church book duplicates of the register weren't made. They may not exist for your area, but in some geographical areas you can get duplicates even back to the 1600s. Check https://www.compgen.de/ for OFBs and other sources of local heritage books in case someone else has already looked at this register.