RIC98 Sorry, I'm afraid I don't know a way to help you with the problems you've listed.
RIC98 The vibrations or haptics were made worse
I personally haven't noticed a single difference on either of my phones.
RIC98 and even more critically: The sensitivity of the 3-button navigation was changed.
I can see how this can be a problem. I just tried on my old phone and could only make the buttons bigger by changing accessibility settings. Might that be a solution? I changed "display size" in accessibility settings, but that makes everything bigger, not just the buttons, which may not be what you want.
RIC98 Is there a way to prevent the crashes when restarting or when unlocking the Private Space?
I don't believe so.
RIC98 Or is it possible to reinstall version 25.08.32 of the Google Play Services?
In short, not any way that I'd suggest.
I definitely wouldn't suggest getting apks from elsewhere on the Internet. I do know that there's some process that the developers use to get a version of Play and or Play Services that has support for multiple locales (if I'm remembering the right word here). All the files for installing older versions of Play Store or Play Services are probably not on the servers anymore. Also, I don't know if other apks that you can find elsewhere on the Internet will even work or be compatible with GrapheneOS's compatibility layer. Another problem, if I'm remembering correctly, is that installing Google Play can only be done by the GrapheneOS App Store app. There's a way to get around that, but I can't remember how right now.
Sorry, not sure what to suggest. Staying on an older version will result in more things breaking as time goes on. Maybe the best way forward would be to update and use the display size accessibility setting?