[QUOTESo for new printer owners without a warranty, I wonder if there is a list of parts they should have on hand to be ready for routine maintenance? ][/QUOTE]
lol - in your case ?
probably my workshop.
You do seem to have more problems than most.
It's basically the controller boards that are going to start getting expensive.
Depending on where you look some prices have already doubled from this time last year.
Stuff is getting expensive on ebay - while not yet changing much on amazon.

But 3d printers are not complicated machines, they do a pretty remarkable and complex job.
But at the end of the day, it's a hot glue gun on rails.

None of the basic components of a printer are going to 'wear' out.
Belts tend to only break if they've been rubbing.

So with basic maintenance, there's no reason the mechanical parts shouldn't last for many years.

with the ender 5 creality didn't get 'clever' or cut a lot of corners.
They just built a large framed standard cartesian printer.
So as long as you make sure the frame stays tight and any rods or rails are lightly lubricated and bolts are tight.
No reason at all anything should fail.

generally the electronics will go before the mechanics. And modern electronics are pretty durable.

In the unlikely event that something does get damaged - pretty much every single part of a 3d printer can be had for next day delivery from amazon.

So if I was going to have a spare of anything - it would be the motherboard and a spare set of stepper drivers - they do go on an irregular basis.
And get one now - they will be going up in price and potentially getting hard to get hold of in the forseeable future.

Plus if you have the exact same board and firmware - it's a pretty quick change over.
If you have to make do with a different board - then things get complicated with firmware and screens and it's generally not quick or easy.