@certainmisuse
Couple of things to help or confuse.
Diver Rite Trans
PAC - is a soft backplate harness. It was designed for shore diving allowing the weight of the gear to go on your hips. The harness is multi piece with plastic clips to allow easy in and out access and adjustment. It does have some back padding which is slightly buoyant (despite what Dive Rite claim)
The Trans
PLATE is a similar harness and set up but this time designed to fit in a hard plate
With both you fit which ever wing you desire for yoru needs.
I own both and use the Dive Rite voyager wing. I'm not a die hard DR person, its just is was the gear most suited to my needs and wants that I could buy locally (I like to touch and feel before buying)
The one piece harness as other have discussed is a one piece webbing with no quick releases fitted to a hard backplate of your desire (weight needs) and again a wing of your choice
I used a 1 piece harness on my Side mount rig..
Some wings require the use of a STA (Single tank adaptor) which bolts to teh plate and sandwiches the wing in the middle and has your Cam bands to hold your tank. It adds approx 2lbs to the weight
Other wings like the Diverite do not require a STA with their center section taking the cam bands. Some people recommend a soft OMS tank stabiliser to stop the tank rocking - I use big steels and don't use one and have no issues.
People get put off by the basic webbing harness and metal plate. It's not uncomfortable at all. The shoulder straps don't need to be tight (you should be able to get a clenched fist between teh strap and your chest under water. Teh single webbing harness takes a little bit longer to set up than teh adjustable ones - but it's not too onerous - there are plenty on videos on Youtube
I don't use the adjustments on my straps other than changing the length between exposure suits. Its less faff than a one piece
For additional bits n pieces I use two pockets on my waist belt - personal preference, My dry suit with its thigh pockets is only used 3 months of the year and I prefer to keep my gear in all the same place between rigs
Because I use little weight I use two small pockets on my cam bands - more weight requires a weight belt or harness.
Now BCD - yes I also own one and use it for teaching OW etc if my students are in Jackets (I know too much dive gear)
I can (and do) dive the same configuration (big steel) and I can sling a pony in my BDC It's just as streamlined and just as easier to don and doff under water. I miss it not having a crotch strap on the surface where it rides up when teaching and I wouldn't use it on my big current sites because it create more drag than my wings. Other than that I can jump from one config to teh next without any thought