Any advice on post-stroke restless?

You can dress dad in an anti strip suit to prevent picking on his briefs.
Ssearch Alzheimer's anti strip clothing on Amazon. Ativan can help with agitated behavior, too, and OCD picking.

Sliding down the wheelchair does come from strokes which can cause imbalance in the body and loss of core strength in general. My mother had a terrible time with slumping over in her wheelchair after a couple of strokes. A chair cushion from physical therapy helped some, or a rough towel on the seat to prevent slipping. A wheelchair w a high,back that means BACK is your best bet, but even then it can get bad to where he's bed bound. The carers just kept lifting mom back into an upright position, over and over, basically.

Best of luck to you

You can try putting a pair of his "old" underwear on over the pull up Depends he is using now.
There are also adult "onsies" that will help prevent him from picking at the brief.
Try redirecting him to fidget with something else. Give him a toy or something else for him to fiddle with.
My Husband chewed on things I gave him teething toy.
You could also try to get him to fold towels, pair up socks.
The block toy Jenga might give him something to do.
No matter what you give him watch if he starts to chew on anything make sure it is safe.

He will not "get it" when you try to correct him.
Redirection is the name of the game.
Try "Dad I know this bothers you how about you work on this project for a bit" Then give him something else to occupy his hands.
Give him time to react, absorb what you have said. It takes someone with any dementia a little while to hear then process what you have said.
But while you redirect him validate what he must be feeling.

Your father won't be able to absorb your admonitions or corrections against any picking and pulling at things. I have seen mittens used, but they seldom work. The sliding will likely continue as well, and if you are giving 24/7 care I don't think your back is going to last long. I think that many of these problems are due to the dementia, rather than the stroke, unless you are seeing a great deal of worsening immediately following the stroke.

I am so sorry. I wonder if it is not time to consider some placement so Dad can be moved more safely.
I hope others have better answers for you than I have.
My heart goes out to you both.

Try setting him up with something to keep his hands occupied - look for adult fidget toys. Have him sort/count/arrange coins, nuts and bolts, a tackle box or something else that might be of interest. If he's capable try small building projects like a birdhouse or bird feeder.
Some people have used those pad you use under rugs to keep from slipping on a chair.