I've noticed that when I turn into a corner quickly or am leaned over somewhat far (not near the bike's limits, I'm sure) that the rear wheel will often lose traction and skid a bit, until regaining traction.
Well, based on that original complaint and your discovery of a topped out hard
rear suspension, I think you have your primary answer and immediate problem. Backing the spring pre-load off to get the kinds of sag numbers mentioned will make a big improvement.
I've taken some liberties to fill in the blanks on your original post, assuming you might better describe the problem as a "skipping" action when you hit a bump in a corner, bouncing the rear, unloading it, and re-gripping when it comes back down. Almost as if it "skips" or "steps out" a small amount. If that's grossly wrong, please say so.
I looked back through some notes I made a time ago when working out the rear suspension on my 500 and found my ratio and static load numbers. I also found an error in a post in another thread.
The mentioned (soon to be edited) 3.5:1 ratio is correct for linkage length
changes, adjusting ride height by modifying the swing arm link lengths, but is a different thing altogether and not correct for shock adjustments. That ratio should read 2.85:1. I'll edit them after making this post in case somebody would use them in the future. The ratio really just helps to not have to keep taking the shock in and out, adjusting purely by trial and error.
For a starting point, remove the shock and back the spring off all of the way to loose. Bring the nut down to just
take the play out and measure the spring's free length. Then continue to tighten the nut, compressing the spring 7/8" from its free length. This should be a very close starting point, if not perfect (1/8" to 1/4" free sag). Install it and check your results. If final minor adjustment is needed use a 2.85:1 ratio for the required change. Wheel change divided by 2.85 is the change required at the spring, due to linkage ratios.
The tire items mentioned are not without merit but I don't feel they are the cause of the exact issue you've brought up here. The lower pressures shouldn't cause what you described. They effect wear but should only help grip, not hurt it, as long as they were at at least 30psi. Much lower than 28 front and 30 rear, I'd say you might feel something if you were more experienced but not at the experience level we're talking here, if I understand it properly. As your experience grows, tires will prove to be very important to you but let that be its own search or new thread.
Access to the top shock bolt requires removing the tail section and using two sockets and extensions, one from each side, to grip the bolt and nut. The lower should be evident looking at it from the bottom. It drops out the bottom.
While it's apart, slide the linkage bearing sleeves and seals out, clean, grease, and re-assemble. Good maintenance item done. Also, lift the rear wheel or set the bike on the ground before to final tighten all of the bolts. That will take all of the play out of them in the normally loaded direction. Good luck.