Theres a lot of decent mountain biking in majorca, but A lot of it is hard to find, and I reckon would be better with a guide & van.
We went for a long weekend a few years ago, and we cobbled together some routes by trawling the internet & buying a guide book from tramontana tours in soller .
A particularly memorable ride was one known as the pilgrims steps, you start high up next to cuber reservoir (having a car/van to get you up there makes life easier) , and make your way down towards soller, via hundreds of steps. It wasnt exactly fast flowing singletrack , but was worth doing just for the journey.
If we go back we also talked about riding the GR221 trail that runs the length of the island through the mountains. It’s probably a 2 or 3 day ride, but I guess you could just pick sections of it to ride. Just google for GR221.
There’s probably more routes on the internet these days than when we went, so just have a look around and load them onto a GPS.