Things to consider: open box stall (with full length butt bar), nonbreakable mirror so she doesn’t feel alone, you have goats maybe one could be a companion for trailer rides and hang out in trailer while you are on the trails.
Some trailering issues can be pain related. Ulcers can make horses very unhappy in the trailer as the movement of the trailer causes sloshing of stomach acid onto ulcers. One bad trailering horse I had (he once pawed a rubber mat off the wall of the trailer and tore it in half) improved on ulcer guard started two days out before any trailer ride but that didn’t totally fix him. What did was removing all forms of grain and grain products from his diet and on a forage diet he became a great trailerer and a happy and competitive GP horse.
Another bad trailering horse improved when I put front shoes on him, seems his feet were stinging and he was becoming anxious about upcoming work once we arrived at our destination.
Don’t give up, keep looking for the source of the anxiety.
I never tried it but one top trainer told me he blindfolded his difficult to trailer horse and she started traveling perfectly and arriving cool instead of dripping sweat. Some horses really don’t like the noise inside the trailer and adding earplugs and/or masking sound with soothing music can help.
Good luck!