The first part of this achievement I completed on a different character. I have completed the rest on another character. I heard this is supposed to be account-wide now so im unsure as to why this is not working. I believe this is why the Blood Gate questline doesnt seem to be showing up. Any help would be great because i just spent 6 hours grinding all this out lol.
Another post about this within the past day. Has some things you may want to compare against.
https://us.forums.blizzard.com/en/wow/t/cant-unlock-zandalartroll-or-maghar-orc/779096
Don’t confuse the achievement “Zandalar Forever!” with the scenario called Zandalar Forever, which needs to be completed in order to earn the achievement “The Final Seal.”
In order to earn “The Final Seal,” one of your characters must make enough story progress in Zandalar to reach that point.
The comments from wowhead might be helpful. They outline the requirements pretty clearly:
The Final Seal - Achievement - World of Warcraft (wowhead.com)