[Leo.G (Leo)]

Definitely easier and cheaper than going with alloy but the 5.5mm wall thickness should hold up a lot better to wear.
The other simple solution I've used myself is RivNuts (rivet nuts). Now that may have you scratching your head but they are available in alloy and steel (possibly stainless steel too, I'm not sure), if you pop them in then drill the thread out where the steel axle is going through they work well as a sleeve/bush to prevent ware on the softer PVC piping and are cheap and very easy to work with.
If you don't have a rivnut gun they can be secured using a bolt and washer arrangement and I believe there's a lot of information on doing exactly that available online.
Food for thought.
Remember to pop them in place before drilling/grinding the thread out and a smear of grease (lithium spray is good and often cheap).
Were I copying your idea I'd probably go all OCD and drill a hole in the shafts for a grease nipple and a small collar on the axle but I have a metal lathe handy. Come to think of it I'm such a lazy b@stard I'd be more likely to give them a squirt of silicone spray or WD40 when the squealing started hurting my ears. I have to be honest. I do like to think I'd do everything properly but sometimes cheap, easy and no fuss wins, only 80% of the time though, the other 20% I'm pedantic (that means my son gets in my ear).
Gotta be honest with at least myself!
I use to be particular about everything until an OCD perfectionist plumber friend informed me when I saw him doing a rough plumbing job "I've had to accept it's not a perfect world"
I was shocked, this guy was the perfectionist of perfectionists.


I told you my son would get in my ear, he's smarter than I am!
He just mentioned the issue of getting two sleeves square to each other for an axle. Now I'm a quick thinker (it's the only thing I can do in any sort of reasonable time frame) and came up with an instant solution, if you use a suitable length bolt (or threaded rod), a sleeve and a few nuts and washers you can align the two Rivnuts and compress them into place together. Then you only have to drill or Dremel the thread out reasonable square.