brent I did look around, but didn't see any answers that specifically fit my issue. The one I was given still isn't, but it worked. Sort of
@Justin LVM had no effect, nor did encryption. It would light up when the passwords matched, I'd click it, then it would go grey again.
@WetGeek Clicking on the list on the left didn't do anything, either going forwards or backwards.
But I did manage to fix it, and it's all kind of stupid.
I looked at the instructions in the post from brent, and since I wasn't trying to put Solus alongside another operating system on a disk, I first just wiped the drive (it used to have Manjaro on it) and then told the Solus installer to partition it and install Solus. Same thing happened. I went back and created an EFI partition according to the instructions on the empty drive, then went back to install. The installer then informed me that Windows had configured the system to boot in Legacy BIOS. My BIOS settings are set to boot in either UEFI or Legacy (Asus Strix motherboard, it has settings for either UEFI, Legacy, or both). So... I rebooted in legacy mode, and now I'm typing this from my newly installed and upgrades Solus.
I assumed the "both" setting would just go with whatever I booted in, so if I booted UEFI everything would just go UEFI. I can't remember what Windows is installed under, but maybe I did set it to MBR when I used Rufus to create the USB a couple years ago. Dunno, but all is well, now. Thank you all for your suggestions.