It may seem like a lot, but if you take the time to read through it; it will likely lead to a solution. Although it's the arch wiki most of the content will be applicable.

https://wiki.archlinux.org/title/Dual_boot_with_Windows

Edit: I just realised your Edit was saying you've got legacy bios! So this is all irrelevant. I'll leave it up in case it helps someone else.

Could it be selecting the wrong SSD to put the boot loader on?

When I reinstalled mint the other day on my laptop with an nvme and SSD (also dual boot) it labelled the extra SSD as sda and the original nvme as sdb, so it was going to try to put the bootloader on sda.

I set up the partitions on the third option (1. Install alongside windows; 2. Wipe everything; 3. Set it up manually) and on the manual setup there's a selector for the bootloader device just underneath the main section where you select partitions to use for /, /home, etc.

IIRC you set the bootloader to the full device (in my case sdb) not the EFI partition (sdb1) and it works out out.

Best bet when installing a second OS is to MANUALLY partition, select the existing fat-32 efi partition for efi, write the boot block to the drive you're going to boot off of, and choose free space for the remaining required partitions.