Hi Kenny, congratulations on your progress!
It is good that you are mindful about the volume, some windsurfers make that step too soon and it causes a lot of frustration, although I understand the appeal of small boards and the adrenaline associated with it!
My personal opinion also concurs with your gut-feel: a 150-litre board would be a more natural transition, and will allow you to progress quicker in the mid-term.
All the best, and keep up the good job!