Tim
I'm not familiar with those sails, but new freeride gear, you can have large gaps as the sails have large wind ranges. My quiver for freeride is 6.5 and 5.5 and there is a big overlap with these sails (I could easily go 7.0 and a 5.0 if I wanted to, but a 7.0 is too much sail for me). Wave/B&J: 4.7 and 4.0.
I think a 4.5 on a 120L would work, but it would be no fun.
I would pick your conditions (8-25 knots) and practice with what you have for the time being. In general, bigger gear is easier to experiment and learn new skills.
If you are able to borrow a fellow locals rig/board/fins for a few runs, this will help you decide what to save up for next season.
Welcome back

PS/ I'm 72kg at the moment. (75kg in winter

)