This comment reminded me I needed to re-lubricate my bike chain, which I hadn't done for something like 7 years, so I did that and took it for a spin. Got rid of the squeaking, noticeably smoother and easier pedaling, but a relatively minor difference.
But yeah, if you can't get replacement tubes, that's a different story, I have to replace those once or twice a year or the bike is not going anywhere.