Huh… I thought Synology had a cap of 143 characters. Are you writing a book? LOL

So if I want that backup to work, it appears I have to rename hundreds of files on that PC

You could zip or tar the offending files. Inconvenient but would work. You could use VeraCrypt or Cryptomator containers which would give you 255 character cap. Using a VeraCrypt or Cryptomator container will let you store files with longer names inside the encrypted volume, but it won’t change the 143‑character limit for the container filensmr itself on the Synology NAS.