I installed a Gnome Boxes Windows 10 VM on two computers today -- a Dell Optiplex 7050, i5 6600T, Intel 530 onboard graphics, 8gb RAM, 128gb SSD and a Dell Optiplex 7070, i7 9700T, Intel 620 onboard graphics, 16gb RAM, 256 gb SSD -- using the 20H2 iso from Microsoft's download center for non-Windows operating systems.
In each case, Gnome Boxes Express Setup (which purports to set up everything automatically) failed, but Custom Setup worked smoothly.
I did not attempt to license either install because I do not intend to use either VM and will remove the two VM's tomorrow.
Several observations about Gnome Boxes Windows 10 VM:
(1) Gnome Boxes default settings for Windows 10 installation is 2gb RAM, 40 gb disk space. While this meets minimum Microsoft requirements for running Windows 10, it is clearly not adequate. Windows 10 realistically requires at least 4gb RAM to run, and a bare-bones install, without applications or data, uses about 32gb of disk space, so 64 gb of disk space is more in line with reality for most users. The defaults can be changed during Custom setup.
(2) I set up the Optiplex 7050 VM to use 4gb of RAM and 50gb of disk space. Windows 10 ran okay with that setup, but somewhat sluggishly when performing tasks that required significant RAM. Performance was equal to or exceeded performance on a bare metal install on my Dell Inspiron 3180, AMD A9 processor, R5 onboard graphics, 4gb RAM, 128gb SSD, so that configuration is workable.
(3) I setup the Optiplex 7070 VM to use 8gb of RAM and 64gb of disk space. Windows 10 ran very well in that configuration, similar in performance to bare metal Windows 10 on my Dell Latitude 7280, i5 7300U, Intel 620 onboard graphics, 8gb RAM, 128gb SSD.
(4) Based on that configuration comparison, I would suggest that anyone intending to use a Gnome Boxes Windows 10 VM for serious work (more than e-mail, browsing, undemanding tasks like word processing and spreadsheet work) install a VM with 8gb of RAM devoted to the VM, if possible.
(5) I did not install any serious graphics programs (e.g. Photoshop) to see how that worked in the Gnome Boxes VM.
(6) The version of Gnome Boxes included in the repository is 3.36.6-stable, which appears to have basic Spice functionality included. I was able to change Windows 10 display resolution and could access USB flash drives without difficulty, following Gnome Boxes Help instructions. USB mounting is single-device, single-use, but it is workable. I've read that installing the Windows Guest Tools for Spice in the Windows VM may enhance the VM in this regard, but I didn't test it.
(7) As expected, I was not able to access Solus 4.1 files from the Windows 10 VM. In this respect, the VM is similar to a dual-drive dual-boot setup. I've read that installing Windows Guest Tools for Spice (see (6) above) and the Spice wavdev daemon from the same source can enable access, but I didn't test it.
(8) I was able to install/uninstall Windows programs like LibreOffice without difficulty. I did not attempt to install any apps from the Microsoft Store because I was running an unlicensed version, but I see no reason why that would not work.
(9) I did several Windows 10 updates in the VM, and that process seems to work fine.
Overall, my testing suggests that Gnome Boxes is likely to provide a workable solution for Solus users who need to run Windows for specific tasks, and is probably a much better solution for someone like @oekorem than keeping Windows as the bare metal OS and running Solus in a VM. Thanks to @jrsilvey for pushing this alternative to the forefront.