I'm a lower intermediate SUP surfer. Can catch waves and get down the line. Currently on a Jimmy Lewis striker 9'5. The stock centre fin is a JL 8' swept back fin. Want to start experimenting with different fins. Off on an adventure next week and really hoping to start nailing turns. Any suggestions or alternatives I can play around with? ( Around 80 kegs if that's relevant.) Cheers in advance.

I use all sorts of fins on all my boards
Yesterday I rode a 12' board in tiny surf, so I threw in a 3.5 " center fin to allow the tail to slide around on turns, since there was no power to rail turn on.
So yeah... get a smaller center, maybe a 5.5" and set up as a thruster.

Also try a bigger single center fin, with less rake (sweep)... it will allow a shorter radius pivot turn

Normally I would advise like Supthecreek to try a thruster setup (rear of about the size of the fronts), but with semi-retro longboard shapes like the Striker, and you seem like a fin novice, you may want first to play with the fin position: try to surf with the fin to the front and to the rear of the box, and move it around (a tool-less 9' FCS fin is very nice for this: you can change the position between waves on the water).

This will make you understand the effect of a fin, starting with the basics.

I have the same board and have experimented with fins a lot. Best combo I've found is the standard JL side fins with a 7" cutaway set with the leading edge 2.5" from the front of the finbox. Very loose but still plenty of drive.

I'm clueless about fins, but as someone who has been working a lot on their turns I'd just chime in and say make sure you're paying a lot of attention to how far back and towards the wave-side edge your rear foot is. Maybe striker not as sensitive but my worldwide (w/ stock quads) can feel really different based on foot position. I think for me it's influenced by the wave, like on a steeper/larger wave I am more distracted or intimidated so I don't get foot as far back or near rail, then I can't crank as hard on bottom turn. Similarly the board feels locked up on top turn if I'm not close to tail pad. For a little while I thought I had a fin issue when it was prob my feet not being where I thought they were -- I think after a session of focusing on foot placement, I assume I can just go by feel on the next one but maybe I still need to actively think about it.

That said I just ordered two sets of Quobbas and I'm excited to experiment. Anyway, good luck with the turns, hope your crank some nice ones.