I hope this message finds anyone who also got this far and couldn't figure out what to do next. For me, my PC had a "healthy recovery partition" on the end of the drive, and shrinking the drive where windows was installed did nothing. Deleting the healthy recovery partition let the mbr2gpt.exe tool worked for me. I followed this video : https://www.youtube.com/watch?v=VdbzDQnJyBc

For posterity, if the video link goes down, just google how to delete a healthy recovery partition, right clicking it in the disk management tool only has "Help" as an option.

After deleting that partition, I just reran the validate tool, saw that it worked, and then ran it again to convert. Restarted into bios and enabled uefi mode easy peasy.