I was last year around Europe for some months mountain biking, including some marathon races. My favourite 2 places are Livigno in Italy, very close to Swiss border and Finale Ligure, Italian riviera. Finale’s best bet is ride around the 24h Finale Ligure race course, otherwise you’ll have to deal with more technical, rocky, steep routes.
Livigno is just amazing. At tourist office you can buy high quality mtb maps for 1 euro only.It’s duty free area as well.
Garda lake in Italy has some good routes, but lots for all mountain freeride type.
Switzerland would be the country to stay. If you go to myswitzerland.com you’ll find plenty of information for national, regional and local mtb routes.
I enjoyed also riding just north of Bratislava, around the Stupavsky marathon course.
Forgot Bormio, 1 hour drive from Livigno. Lot of 6 maps including Livigno very cheap. St Moritz is also close to Livigno. DId a couple of routes linking both places.
If you like marathon racing, you won’t be dissappointed.