I would shorten up the rear end of the receiver tube until it is flush to the face of the rear cross-member. That way the tail gate can drop to truly vertical. As it is now it will hit the hitch and try to get bent there should anything push on the tail gate while it is down. As a bonus this will move the std. cross pin hole a little further forward and away from the front face of the rear cross-member making it easier to R&R the cross pin.
Consider some sort of tail gate latch that pulls up tight to reduce the number of rattle points. Something like this:
McMaster-Carr or this:
McMaster-Carr McMaster is probably not the lowest cost option, but that'll give you an idea of what I'm thinking of.
I think that after fully tacking everything that I'd weld all of the seams full-length using the back-stepping technique on the inside of the tub, but only use ~1" beads on the outside. Then prep and use seam sealer where you didn't weld on the outside before painting it. Especially on the underside and in the fenders.
Look over the whole thing with an eye towards "where can dirt/ice/salty slush/etc. build up in a ledge or in a crevice?" and see if you can alter the design slightly to make that ledge etc. go away.
Also think about where the wiring is going to go. Inside the tube is easy for protection, but that has it's share of problems. Now you need holes into the frame where junk can get in, where the wires can fray on the edge of, and where it is difficult to fish the wire through in the first running of wire. I'd look into other options and I've none that I really like to suggest. The closest to likable, in my eyes, is to use trailer light cable for the whole thing with dedicated holders. Like:
Trailer Cable | Trailer Wire Making connections under the trailer is the toughie. Maybe inside of sealed J-boxes with liquid tight cord-grips? I dunno.....
At the very least use adhesive lined heat shrink over the terminals. Can get them combined like these:
Heat Shrink Butt Connectors or get the heat shrink on it's own and use uninsulated terminals.
The bane of trailers is always their lighting, and it is the grounding that is nearly always at fault. I have had excellent results from NOT using the trailer frame for the grounds. I run a dedicated ground wire for each load. Sometimes that wire is connected to a mounting bolt because the unit's mfg didn't provide a dedicated ground wire and intended the unit to ground thru it mount(s). That's OK, just keep running dedicated ground wires.
Even if you're only thinking to bed-liner the bed and not the whole thing, give some serious thought to also doing the inside of the fenders. It will keep the big rocks from denting the fender panels on the backside & causing the paint to crack in a star pattern. If you're going to paint part of it, do that first. Might consider also asking the bed liner folks about masking off where people are likely to step and putting some liner there for traction and wear.