I have two BP’s neither are Halcyon (FredT & Deep Sea Supply) and I’ve never felt the need for a back pad or padded harness.
If the weight belts normally used to make a harness cut in to you when wearing a 3 mil try making a harness from a car seat belt, they are softer but can be hard to slip into.
Some make wings don’t require a single tank adapter, two rods in the wing keep the tank from flopping.