No problem doing that! We have a similar setup. Although if your app is primarily web based, it may be better DX to just use server actions and server components instead of a separate C# backend.

For us we're 60% mobile so a separate rest service is easier.

We mostly use server components for calling our custom backend which at time feels a little strange

Where would you even write Prisma to write to the Database or retrieve

Since Backend C# cannot run with prisma

Or can you even write Prisma in c# ?