Some thoughts..
OP..we're walking down a similar path.. in Wisconsin, of a certain age, bikes, kayaks, 12 foot box popup previously(and still..for the time being)
We acquired a used 6w x 12l x 7h Wells Cargo trailer a few months back. It has a ramp rear door. There's a good case to be made for either type of rear door (ramp or barn). Unless you're loading some sort of motorized vehicle, either will work fine. We have a ramp as that's what was on the trailer. I like it so far.
RE: bike and kayak storage/transport..that was one of the main attractions to a cargo type trailer. I ride high-end steel or titanium mtn/road bikes and cringe when traveling in a driving rainstorm with the bikes on an outdoor rack. Driving water, and in the winter salt water, into every nook and cranny of a bike & components..I hate it. While bikes can easily be locked onto a bike rack, offering some security..though not a lot, kayaks on a roof rack are harder to secure. If we're traveling and stay in a hotel overnight we can now lock everything in the trailer. Bikes always ride in the trailer. The wildcard though is you still need to transport bikes or kayaks once you set up camp if you plan on biking or kayaking somewhere distant from camp. Something to consider. I still have kayak racks on top of the tow vehicle and a bike rack plugged into a front of truck hitch for those occasions.
RE: trailer interior setup.. fwiw...see what we're doing below. (trailer not built out yet..it's a work in progress).
Our trailer came with E-tracks, so we're taking advantage of them. Very nice option. If I were to start from ordering a new trailer, I'd consider having tracks installed on the walls near the ceiling, mid or low-of-mid wall(allows bed platform mounting with fold-down-bed platform-hinged legs), and near the floor. They offer lots of versatility without taking any room if you don't need a lock-in accessory.
trailer:

Fold-down bed platforms as designed(hinged off the e-tracks):

As built:



We've used the bed platform and is brick-solid stable. Folds up and down in minutes and takes up very little room when down. We use a queen air mattress on the platform. Very comfortable (use reflector sheet on top of mattress) and folds small when not needed.
With regard to bike and kayak storage. Initially we planned on loading the kayaks on the (setup) bed platforms and securing them, but this made bike transport more difficult and eliminated the option (pretty much) of carrying 4 bikes (two road, two off-road). After some thought and experimentation we came up with the following. It allows easy kayak loading, takes up very little space, and allows us to carry 4 well-secured bikes. Both kayaks fit under one side of the bed when setup. The bottom kayak sits on foam blocks. The top kayak will sit in a rope or strap cradle just above the bottom one.
The original front cabinet design (drawers on left), changed to allow kayaks below bed platform (drawers removed and made into an open storage cabinet):


Bikes are secured with fork mounts. 2x4s are secured with e-track mounted cradles via quick-nuts and knobs:




With respect to trailer size..you have a small-mid sized truck. You'll appreciate the easiest towing you can get. Ours is 6 x 12+v-nose x 7. Standing up is a BIG plus..I'd go with a 6'6" minimum inside. Six feet wide..same width as my truck (Sequoia)..it's nice to see easily with the mirrors. Single axle is easy to back up, tandems turn harder (been there) but they tow nice.
Regarding interior build.. given the preferred small size of the trailer, we're going with a fairly minimal setup. We don't see any hardship in cooking outside, under a nylon tarp or screen shelter, if needed. We'll have a one-burner propane stove inside(tucked in a drawer when not needed) to make coffee occasionally. We won't have any sinks, toilets, showers..all that is outside, again, without hardship on our part..at least it never has been. All that stuff takes interior room and maintenance(been there too). Showers..if you've ever taken a shower in a small bathroom you know how much humidity is developed. Some folks here have had mold issues inside..we don't want that much moisture in the unit. Parks have reasonable showers or we take sponge-baths. In larger trailers, it may be different...good venting is needed for sure.
Everyone has different needs or desires..you net out where you need to be.