I've been through a lot of Garmin devices and mine always have battery degradation before the USB port is in any way non-functional. Especially with USB-C on the newest ones, that really shouldn't be happening.
You are correct, the last time I had to replace my 530, the 530 was on sale due to the 540 coming out so I bought the 530 on sale and haven't thought about it since. For some reason I thought I bought a 540 after comparing the product pages for so long.