I think weight issue is border line, and you probably should get a bigger trailer.... What bother me somewhat in picture is axle placement of trailer and a lot of weigh of tractor forward, kind of wonder how much tongue weight is on hitch?
Also some states (like Kommuist Kalifornia) require four point tie down for "vehicles" on trailers....
Really weak link here may be ramp, 1> It may prevent you from balancing load (forcing load to far forward)... 2> it will probably bend and fail at worst possible time...
As for changing out axle and adding brakes, also consider frame material and hitch size, it may not be adequate for additional weight... Might be better to sell trailer and replace with larger size....
I move tractor and Jeep and Dune Buggies around on a dual axle "car carrier", but it also came at a time when I was towing almost ever weekend to autocross races, and my opinion is a trailer can not be to big (within certain bounds)... And yes on brakes, had two incidents in same day about 30 miles apart I was really glad trailer has brakes.... Both times I was cut off when running about 55 MPH and if it were not for added trailer brakes both incidents probably would have wrecked truck and trailer and race car on trailer.... And as rear bumper for offending car disappears below hood of PU and anti-lock brakes are in full action and trailer brakes are squalling, your first though is how good are tiedowns on car on trailer, is car going to come forward into PU bed?... Both time you probably could not put a piece of paper between car and my PU bumper but we did not touch... Towing in is whole different dynamic and most idiots on road are not aware your driving situation is different from theirs... Love the idiot at on ramp, screaming down ramp and going to tuck in behind my truck only to realize there is no space for him as there is a trailer on back of truck....