True, larger tires will make a difference. If you're thinking of putting bigger ones on the trailer, you'll have to bear in mind the height of leg lift increase required by the horses to step in/out of the trailer and decide if that may be problematic for them.
None of the horses I trailer should have a problem with the higher height. And since its a stock trailer I can turn them around and let them walk out head first and not make them back out of a steep drop.
Alright, talked to my dad and brother and they're going to check whether it is an over mount or under mount (whether the leaf springs mount over or under the axel) and they might be able to flip the leaf springs. They think blocking the trailer, if it is done right, would be a safe option as well.
Oh dear. I`m afraid to say it`s not looking that good. The back axle is taking more weight than the front one on the trailer which will mean uneven wear on the axles and potential stress damage to the rear one. If the heaviest two horses were in the front that may help but I think in the end it just needs to be levelled.