I live here/there….. Not done much riding on the coast but spent plenty of time there. As above, it is rock and gravel. Plenty of forestry mountain tracks, donkey tracks and fire roads everywhere…. As long as you are respectful you pretty much have freedom to roam….. Think about where you are going though and make provisions. It is hot and dry in the summer, heat from the sun above and rock below can get on top of you quicker than you think….
It is mountainous so can be very upy downy, mainly the Velebits on the mainland, the islands less so. Roads will be busy so stay clear, all tourist drivers who have no idea where they are going so are pretty aimless.
I’ll speak to some local Croat mates and see if they have any info on routes near to where you will be staying.
Get inland a bit, especially Plitvice lakes, and you will find more ‘temperate’ single track. I ride around Sljeme above Zagreb, it is great and the buses have bike racks to take you back up to the top…. You’ll only want to ride up it once.