Seems really foolish to me to have a final quest be a dungeon. Once people have moved on to an X-Pack, it becomes impossible to finish a questline that ends with a dungeon. Either add bots like in the newer starting areas, or change the content to not require a dungeon run because nobody joins queues for older dungeon content and you just wait forever for help that will never come.
BFA dungeons can be easily soloable, just need to level up a little more.
You can also make a post in the legacy raid or custom group finders, someone there may be willing to help. I’ve done so a few times to help out players.