Thanks for playing
Honestly, I think we used the basic FPS recipe from KidsCanCode. We put that in on day one and it worked fine with Jolt. Never looked back. Jolt doesn't support an endless ground plane, so we had to switch to boxes, but that was the only thing I even noticed with respect to Jolt.
That said, we don't have a lot of heavy physics. Plants are static bodies. Enemies are CharacterBody3D's. We switched some of our bullets from Area3Ds to RigidBody3Ds, so that might be a bit taxing. Jolt seems to handle it well, but I suspect the default physics engine would be ok too.
The tweaking was all about recoil and reload speed, quantity, damage, animation trees, blending animations, and animation timing.
I had trouble getting bullets to aim at the reticle, so I ended up putting a marker some distance down the raycast and made the bullets look at that before moving. (Which caused us all sorts of problems when we realized that plants and enemies were shooting at the player's target as well, because of our inheritance and me messing with the wrong script.) ((Parents, never let your kids give targeting or aiming behaviour directly to bullets. That code should have been in the gun itself. Lesson learned.))