BIOS recognizes the UEFI so we have a conflict as explained above. Whether you choose grub or uefi, you gotta make it all right in your BIOS before any install...or choose UEFI install...
...that all that said the title is misleading here. "so then I rebooted, but was unable to find anything but "Windows boot manager" in the (f12 opened) boot menu under secure boot (and it showed legacy options too)" clearly says you can't find the bootloader or boot solus. your title says upgrading secure boot.
SO like Buzz I feel like I'm missing something.
I only have three observations:
1) reinstall making sure your bios is dialed in to Legacy first. Laptops are bat**it crazy. CSM on or CSM off it does not matter. Only one will work and there is no consistency.
2) If Solus nuked WIN and installed itself how do you still have a WIN bootloader? Like Buzz said ^^ I format the disk before I let Solus (or any distro) format the disk in install. superstitious but it's always worked..
3) I would suggest go back to help page and type chroot and restore the bootloader there (it will work) but only if you have the legacy/uefi thing cleared up and after you have the did-I-make-my-bios-ready?-thing cleared up.
Missing bootloader is a headache. Go back thru all the steps Buzz mentioned. At the end of the day for any fresh install you have to 1) make it right with target disk and 2) make it right with the BIOS. Not sure any distro's installer can make sure your BIOS is copacetic.
2 cents