I remember a long time ago (maybe close to a decade?) I was looking into this a bit because I was interested in iPhone dev, and the requirement that that toolchain use Mac hardware was hard for me to swallow. It still seemed fairly hacky at that point, and ultimately I wasn't willing to shell out hundreds of dollars for a dev license and hardware just to see if it was worth it and I wasn't willing to go that far into the weeds on a commercial closed source platform where any problem I might have would likely be ignored with prejudice.
It's good to know they support some form of virtualization now. Do you know it they support any of the consumer grade virtualization systems? Even if I had to pay for a full macOS license (I think it wasn't nearly as cheap then either) and a developer license, that would have probably been much easier to swallow, and could have led to more hardware investment from me later if things worked out.
ESXi has a free license, but it is a type 1 hypervisor (basically an OS that just runs VMs).
VMWare Workstation can be patched to run vanilla macOS installs too, but unless you're running it under a boot camp'ed Windows on real Apple hardware, it's a license violation: https://github.com/DrDonk/unlocker
You can't buy individual macOS licenses these days, as far as I know. Apple hasn't charged for macOS or updates for at least a few years now. Their license text includes references to volume licensing, but I guess that's for very large-scale companies with direct contacts.
It's good to know they support some form of virtualization now. Do you know it they support any of the consumer grade virtualization systems? Even if I had to pay for a full macOS license (I think it wasn't nearly as cheap then either) and a developer license, that would have probably been much easier to swallow, and could have led to more hardware investment from me later if things worked out.