I figured it out!
So what I did is, i used the knife tool to cut an approximate area around the cracks and seperated it from the rest of the mesh.
I sculpted the cracks into the seperated piece which means that no matter what I do, I won’t manipulate the rest of the mesh with the sculpting tools.
After I finished sculpting the cracks, I put the seperated mesh back into place, adjusted the vertices to match up with the original mesh and done!
At first I tried to reintegrate it, but that just led to many other issues, especially when applying a subdiv modifier. So I left the sculpted part of the mesh seperate but since they match up perfectly, all is fine.
This way I have absolute control over my entire mesh and can unwrap the entire thing almost perfectly.

Thanks everyone for the help! 🙂
//edit: Yeah I know the UVs around the frames of the gravestone are not done yet. I just did a quick and dirty cube unwrap, still need to adjust those UVs. But all in all it works perfect.