Running Antix on Legacy mode (failed attempt – can no longer boot in).
UEFI is okay.
Probably had something to do with installing Grub to MBR (instead of PBR?) or not creating a new partition table first.
I may be trying to read too much into your post, but I am not sure what you mean.
Are antiX and Windows10 both installed in the same computer (or is antix running live)?
Was windows10 installed on UEFI or in Legacy mode? (You should install antiX in the same mode as Windows, if UEFI, both in UEFI mode)
Did you originally install antiX in UEFI or in Legacy mode?
Are you able to boot in to antiX on UEFI mode but not in Legacy mode? Is this live or installed?
When trying to install antiX to UEFI, was there an EFI partition already there (created by Windows)? Did you first disable Secure Boot from the BIOS before booting into antiX?
If you did, as you have already installed antiX, try booting the live USB in UEFI mode, upgrade the system (Menu > Applications > antiX > antiX Updater).
After that, reinstall grub to UEFI
Menu > Control Centre > Maintenance > Boot Repair > Reinstall grub (EFI or to ESP)
If this works, the in Boot repair, select Repair Grub so that it updates the entries for antiX and Widnows.
Then, reboot and your antiX system should boot properly without needing to completely reinstall it.
If no matter what you do you cannot install to the EFI partition, then I am not sure how to get it to work with MBR or PBR if the BIOS manages the Boot order (probably having to change the partition boot order).
Let us know if the EFI reinstallation fails. Maybe someone can point to a good solution.