Are you staying in Barcelona? Do you intend to return to Barcelona after your visit? If “yes” to those two questions, I recommend taking the train and gondola up/down. Really easy to do and then you won’t have to worry about renting a car and parking.
It is a great time up there. One of our favorite things we did during our 2 week Spain trip in 2019. The monastery and surrounding area is breathtakingly beautiful!!! Hiking is quite nice and rewarding as well. If you do take the train and gondola, the train back to Barcelona can get pretty full.
I don’t think a tour is necessary. Usually I like to know the ins and outs of everything, including the history and fun facts. But we didn’t do that for Monserrat and had an outstanding time with our self-guided hike and monastery visit.