You can try re-positioning the rear axle washer to get a bit-more tire clearance.
https://www.vmaxforum.net/threads/rear-wheel-axle-washer-swap-fit-wider-tires.35953/
You need how-much clearance on the tire? Do the washer swap first, and see how-much (if-any) additional room you need. If it's just-barely 'kissing' the driveshaft, at a narrow point of contact, perhaps a touch of sanding of the tire will clearance it sufficiently. I'm not telling you to sand an inch-wide band, down-to the tire cords. Probably a better choice is to mark where the interference is, and to determine how-much space you need-to gain. A bit of heat and some persuasion with a ball pein hammer (yes, you're going to have to repaint the swingarm where you heated it) should gain you whatever space you need.
I think most people on-here who have gone to an 18" rear wheel/tire haven't had that issue. I have a machinist make 5-1/2" X 18" rear wheels to use the stock splined hub, and I haven't had an interference problem with 170 or 180 mm width-tires, in a 55-series aspect-ratio. How-wide is your rim inside the tire flanges? If it's 6" I suspect you will have an interference without clearancing your driveshaft/swingarm.
About your rear caliper, you have an opposed-piston caliper, one of them. I think your post is just a typo: "...my calipers...". You may have a sticking piston or pistons, probably due-to corrosion on the caliper body and the pistons. The pistons should
never be 'tight' on the rotor, unless you have pedal pressure on them. Probably time for a disassembly and cleaning of the rear caliper, a flush of the rear master cylinder, brake line, and caliper, and replacement onto the bike.
I am considering another possibility. You may-have put pressure on the rear brake pedal, while the caliper was off the rotor, and now the pistons are too-far together. You need-to push them apart, so the caliper can easily slip-into place. Then you tighten the bolts and the axle nut, and pump the rear brake pedal a few times to move the pushed-out pistons inwards towards the brake rotor. If you
don't, the first time that you try to-use the rear brake pedal, it will bottom-out against the stop, with no braking, and if you don't have time to pump the rear brake pedal enough to restore your pistons to being just-off your rotor,
no brakes!
I recommend doing a 'reverse-bleed' for your rear brake. It's much-quicker, and more-effective for removing any air in your brake system. You can use the same technique on both f & r brakes, and the hydraulic clutch. No-need to tie-wrap any lever (clutch, brake) or pedal (rear brake) in the hopes that air bubbles will rise-to the master cylinder, because you won't have any bubbles in either the f & r brakes or the hydraulic clutch, if you properly-perform the reverse-bleed.
https://www.vmaxforum.net/threads/reverse-bleeding-clutch.47305/#post-472105
