For the OP's purposes, add one more vote to the NO BLOODY WAY on 7k axles. Reality is the "farm yard junk" that Dexter sells is in so far over its head that loading to rated max is going to have very short component life. Really strange they would install far more tire than the axle could ever hope to carry as quite a bit more $$. Sadly, the brakes will still be totally inadequate for much more than a push type lawn mower on the deck.
Most trailer manufacturers use the hitch weight of bumper pull at 10% as part of the trailer GVW - and I consider that appropriate and correct. BUT: with 14k worth of axle on a 3500 lb trailer in bumper pull configuration, you would have a GVWR of only 15,400 thus a net payload of only 11,900. The OP didn't mention what his required payload was, but given the same trailer with 8k axles you could realistically expect a GVWR of 17,600 for a net of 14,100 - far more useful (I see a fair number of track skidsteers around the 14k level these days). I would suggest having a similar trailer built by same supplier, but using 8k Lippert axles and elec/hyd disc brakes.
Regarding trailers being level with typical spring suspension: The travel of the little balance beams of very short leaves of trailer springs is so short, a trailer (ESPECIALLY tri-axle) will indeed have weight distribution issues if not dead level. No, not a drastic as torsion axles, but no where near what a proper walking beam can do (that IS 100% equalizing). RV based trailer market suspension is just plain awful for anything but ideal light load conditions.
Just a comment on tires: the OP's example of the 17.5 rubber is extremely conservative (and good) but the comments about Chinese rubber needs some reply. Around here, 235 80 or 85 load range G rubber is on hundreds if not thousands of stock trailers. I built a 26k semi with 3 x 8k (disc brake of course) years ago when the only tubeless Gs were Goodyear 159s. Absolute super expensive garbage (and IIRC killed more than a few people on RV use). Badyear "upgraded" to the 614 design when the lawsuits came in, and then they had even more expensive garbage (would lose one every few thousand miles without fail). MANY encounters with rural tire dealers along the way and the managed to convince me to try some Sailun rubber on this trailer (usually around 6.5k per axle). I tried a pair on a double 614 failure trip thinking I would just rip them off when I got home. They are still on the trailer several years and tens of thousands of miles later - along with 4 more of the same thing. It wasn't until mentioning this to a Chinese engineering prof friend of mine that he told me his team had designed and built the equipment that made those tires (in a HUGE government sponsored effort to improve domestic and export tire quality). Ask around a lot of ag tire shops before jumping to ANY trailer tire conclusions.
