I know because I tried to do this with a 2018 Mac Mini. Aside from the very lackluster GPU performance, it's a pretty speedy machine.
I had a fairly generous budget for a new computer. 32GB of RAM was about $200 from Crucial. The external SSD storage options were pretty crappy. Any sort of TB3 option was a few hundred dollars to start. I ended up settling with a USB3 2TB HDD. I then had to copy my Photo and iTunes libraries over to it and re-point the apps to the right spots.
For the external GPU, you're looking at about $500 at a minimum. And then you have to find a place for that to sit, and it's essentially the size of a small ITX PC case.
I ended up returning the Mini and just breaking down and building a vanilla hackintosh. A 8700K/32GB/1TB SSD/RX 580 ITX box with quality parts that's all in one small case (Fractal Design Define Nano S) in the same desk footprint as just the external GPU enclosure, and it's not a thermally-constrained dongletown.
Would I recommend it to anyone? No. But after an initially scary couple of hours, I have for all intents and purposes a normally-functioning macOS box that is noticeably quicker than the Mini for the cost of just the base Mini -- sans all the other junk I had to buy. The macOS install is vanilla, with SIP enabled, and no janky kexts in the OS install - NOTHING custom in the OS install. The bare minimum shoved in the EFI partition.
Everything works (sleep, audio, iMessage, etc). It was marginally more trouble than getting Windows 10 installed. Less trouble than getting Ubuntu installed.
God, it's glorious.