Jevy, nice build and I can share the pain of a ruptured disc surgery. Mine was done in Nov. and just now able to return to work. Some thoughts for you to consider in no order.
The marker lights in the lid are all amber, shoudn't the rear most ones be red?
On the taillights, a little silicone where the plug goes into the light will help seal it from moisture but will also keep the plug from coming loose.
On your trailer tongue, can you drill more 5/8 holes so you can slide the tongue shorter if needed? Would make garage storing easier and might work better on a tight trail.
Keep an old set of your wheel bearings for spares; clean them and grease them so they are ready to use; and bag them in a ziplock.
When you wire your trailer tongue: put a quick disconnect plug on both ends of the tongue so you can remove the wires when not in use. They always get cut or damaged when left on and the trailer is parked no matter how careful you are. Mine wound up being squirrel chew toys. Store it in your tongue box.
Along the tongue, you can weld a chain link now and then with a 1/4" cut in it to hold the wiring in place. Just tuck the wiring in when you use it.
For lighting inside the tongue box and lid, those led touch puck lights like Harbor Freight and hardware stores sell work well. Battey powered and last quite awhile/ and no wiring. Just stick on. I do prefer the ones that roll/tilt a bit for adjustability. ( A white lid or interior really helps with lighting)
Safety chains. I like to weld two chain links to the frame, and then use a quick disconnect chain link to attach the chains with. That way, I don't leave an easy way for someone to steal the trailer even if I remove the tongue. And, I can use the chains to padlock up whatever if needed-i.e. a generator or such. Always find a use for a couple 4' chains. If needed, locking the trailer to a tree is easy enough with a padlock or two. I use 5/16" chain so I can get a bolt or padlock through it.
Agree on the no permanent water tank idea. I like to just carry water jugs and have the portability and ease of filling. I have never stayed out long enough to need anything more than a "bird bath" for a shower.
Tailgates. I love a tailgate; however you lose alot of stability when you have one with a roof top tent. The tub flexes alot. If you go to one, plan your lid support strut mounts ahead of time.
Fridge slide out. Could you built a new tongue box that would encase your fridge and slide out, saving to cut the tub up?
Hilift mounting. Two 1/2" bolts with winguts and washers will secure a hilift whereever you can fit it. Short cable to lock it up. Just put them through the I beam of the jack.
I will quit rambling for now, good luck with your back recovery and builds.
