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Cheap SBC x86-64 ?
(programming.dev)
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Buy a used 1l mini PC like the Lenovo thinkcentre tiny (or similar Dell or HP models). You will have to spend a bit more, but you could get one with a pretty capable Intel CPU from gen 7 or 8 upwards for virtualization with Proxmox.
If you're really just looking for a x86 SBC, get a Radxa with an Intel N100 or something similar. They sell at about your price limit with low specs on Aliexpress:
https://radxa.com/products/x/x4/
I've bought some refurbished 1l PCs in places where others might have bought RPi. Drawbacks are more energy consumption and, generally, no GPIO (at least built-in). Also bigger in size. I have one Lenovo M600 which has four serial ports (DB9) and one parallel port (DB25) on some kind of extension module. Because of the module it also has more space for cooler, thus making it passively cooled.
Fast it ain't, but does fit the bill.
You might even be able to use the parallel port as basic GPIO, especially if it's on the I/O bus and not some sort of PCI adapter.