you got the reparability totally wrong. if the motherboard fries, you have to replace basically the whole device. In a desktop you just replace the motherboard and keep the CPU, GPU and RAM.
you can obviously plug an external HDD, but can you upgrade RAM? can you upgrade to a better CPU or replace your current CPU without replacing half of the phone components? No, you can’t.
and yes, Android uses the Linux kernel. But very few manufacturers release the kernel’s code. No upstream kernel support makes it quite hard to keep updating after the manufacturer stops, even for custom ROMs that have to keep using the kernel as a blob, which eventually becomes inviable.
And if you’re already limiting yourself to devices supported by LineageOS, you’re discarding 90% of all phones, which let’s be honest, if the primary reason to use a flawed OS to self-host was to recycle hardware, you’re discarding most phones anyway so not a great reason.