The "RoonServer shared folder will store the database. Roon Labs recommends using a SSD for this. So it makes sense to create the shared folder on the SSD (if available).
I can’t really give an answer here. This depends a lot on the use case. If you want use lots of DSP instances or have a huge library, the Diskstation might not be up to the task. I’d do a test run with Roon Server on the diskstation.
I initially used my Synology DS718+ as my Roon core. The Roon database on rotational drives. For the most part it was sufficient.
I only started to run into performance limitations when I added room correction to DSD128 or higher.
I am currently on an Sonictransporter i9.