From this post, I got the idea to remove and replace the wheels altogether using a hacksaw.
Here’s what it looks like right now. My problem now is that I can’t remove the leftover metal shaft from the caster wheel.
I’ve tried using some lubricant and hammering it with a screwdriver but it didn’t budge.
Once you take it apart, you’re unlikely to be able to buy new wheels. Better to replace the whole unit - I did this just last week with an office chair.
Pull the entire black part out of the gray part but don’t throw it away yet. Bring the old castors to the hardware store to check the size and shape when you buy new castors. In Australia, new ones are sold like this:
https://www.bunnings.com.au/move-it-50mm-black-hooded-office-chair-castor-2-pack_p3940114
You should be able to find something similar where you are.
Can I recommend rollerblade wheels? AKA inline skate wheels.
You can upgrade while repairing.