Couldn’t you get a used PC? They can be extremely good value for the money. With an Nvidia 1060 or better, even an older PC can deliver decent performance under darktable – my Nvidia 1060 is 5-10x faster than my Ryzen 5 3600X. See here for a recent performance comparison (of a 2019 iMac, M2 Mac Mini with low RAM (8 GB), and my home-built, 3-year-old PC using an 8-year-old Nvidia card): Export times on iMac and Mac mini with diffuse&sharpen