Fenders are a federal mandate, so you’ll need them. Maxxis has 13” and 14” sizes. Some of the bias-ply stuff is really narrow, though, so you’re kind of limited if that’s what you have. If that’s your setup, then, just buy new tire/rim combos off the shelf and keep the others for spares. I had a 5’X8’ with bias-plies on it. I went to the the local farm store and bought a premounted tire and rim for a spare. When not in use, it’s easy enough to just remove them for storage and leave the trailer on blocks. I pulled that thing all over central and southeast USA, without problems. I kept the tires inflated to their maximum and kept my speeds below 65mph.
Your post wasn't clear to me. I understood it as going to 10 ply and then further going to 18" with the 10 ply. Either way, same ply/different diameter or same diameter/different ply...you will notice a difference in any harshness. We're communicating from two different orchards... Consider your truck weighs at least 7000lbs empty. My needs are for a half-loaded, sub-2000lb, tandem axle trailer weighing half as much. Es for me are overkill and unnecessary harshness, regardless of profile. To exaggerate my point, let's say a commercial G rated tire would be even better for your application than an E. Would you then say I need G tires, too?