Serious stuff, but your rehab program will determine much, the fusion area won‘t move much but the areas above may depending on their level of degeneration, be well advised with regard to what is consider an appropriate range of movement above the fusion.
There are alternative options of pop up and even alternative craft, some guys are finding SUPS with abdominal core strength improvement a way to go. ( For this be aware of rotational stresses on the lumbar spine)
a lot depends on other areas of spinal, hip and knee degeneration as well. Get fully assessed and look for advice from a sports physiologist who is aware of surfing/ better still surfs themselves!
Talk further and ask more questions
Cheers Jaffa

I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷