You can kind’a work around the lack of RNP approaches with managed ALT mode. When on final appr ALT, dail in the minimum alt (according to charts) and activate managed alt. VNAV should follow the glide slope quite good.
You can activate approach in the MCDU some miles before destination. No need to activate it right after takeoff.
Also important: when in cruise, you’re not in managed ALT. This is normal. You can activate it again, when starting your descent.
A second way to work around the RNP glide path is to go to TRK mode in the FCU and use the according descent angle. On most of the approaches it is around 3° down (see approach charts).