Hmm, that's quite an interesting one. The:
Failed to start MokManager: Not Found
Leads me to believe that this is indeed something Secure Boot related (or rather, the system looking for files related to secure boot that aren't being found). Given that you were able to boot it initially, and then it stopped after booting back into Windows, I suspect some shenanigans here possibly from Windows. When you were freeing up disk space, did you use Windows' partition manager (I think its just called "Disk Management") at all and modify any partitions?
Just to confirm, is it just Mint that doesn't boot, or does Windows also not boot? One of the nicer things about UEFI is how you can have multiple boot loaders installed, where as back in the (regular) BIOS days with MBR, only one boot loader could be installed.
That is all to say, try entering your PC's boot menu (so the same menu you initially used to boot the Mint USB), you should have multiple boot options from your PC's drive - though Windows' boot loader usually identifies itself as "Windows Boot Manager". If you choose that option (or if its not "clickable", you should be able to switch the order so that its first - then reboot normally), does it at least boot back into Windows?
Strange! I quite like Ventoy, but I was a bit cautious to recommend it originally - I've seen some distros just flat out not work with it (I just picked up an old Thinkpad and Bluefin would not work via Ventoy for example), and while I assumed Mint would probably work, I couldn't be sure.
I'll definitely remember that for the next time I see someone installing Mint though, so I appreciate you letting me know what fixed it!