Bathtub drain and w&O overflow connection
A trap adapter and slip joint connection is what is traditionally used, if your waste and overflow is tubular PVC.
If its Sch 40 PVC, then use a no-hub band clamp to join it to the PVC coming up from the slab. This connection is varmint resistant, and a fernco would not be. The band will also help stabilize the joint if there is slight misalignment.
I would definitely leave an access panel if at all possible. If you use slip joints, then the access panel is required by most model codes.