Windows 10 MBR boot - bootrec /rebuildbcd says "requested system device cannot be found"

During the cleanup after building my PC I managed to break my Windows installation and I can't figure out how to fix it without starting over.

I have a Samsung 960 M.2 SSD in an Asus Strix X370-F and 2 sata HDDs. I've installed Windows 10 pro on the SSD from an old DVD via a USB DVD drive. I've accidentally booted the DVD in legacy mode instead of UEFI for the install, and it formatted the SSD in MBR. For some reason it also formatted one of the HDDs in MBR and made a 500-ish MB system partition on it.

After setting up everything, I nuked the HDD and reformatted it as GPT with a single partition, deleting the system partition. So now I have an MBR SSD with the OS and a recovery partition and 2 GPT HDDs with some user files, but nothing related to the system.

The PC comes up with a blank prompt. I tried running the boot recovery from the DVD both booting it as UEFI and legacy but it did nothing. I tried bootrec /fixmbr and bootrec /fixboot, they completed successfully, since then I get missing bootmgr at boot. I tried bootrec /rebuildbcd, it finds the Windows install but then says "requested system device cannot be found". I tried the suggestion to export and delete my bcd first but that failed too, I don't even have a boot folder.

All guides I find for next steps suggest to create an EFI partition but I can't do that on an MBR drive. What can I do instead?

Thanks a lot!