Looks like you found the mechanisms available in the site. I believe the requirements for reputation within the tag is designed for sites with a wider focus than we have here, like computer programming (Stack Overflow).
Even if you had the required answer score on the tag, it would likely take a long while for enough other users with the required tag answer score to find the tag synonym suggestion and approve it.
So, I'd like to suggest the alternative of getting a moderator involved:
- If you can suggest a tag synonym, do that first; it makes the rest easier.
- If it's a completely non-controversial tag synonym, flag a post with one of the tags, give it custom text, and ask one of us mods to create the tag synonym. Or ping a mod in chat.
- If there's likely to be any kind of controversy about it at all, open up a meta post.
Since you opened up a meta post and I can't imagine a likely use of disc that isn't about disc brakes, I created the tag synonym and also "merged" them.
The merge means that in addition to any future disc turning into disc-brake, the disc tag was removed from all 25 questions that had it and disc-brake was added to 19 of them.
While I was in the synonym maintenance area, I noticed a couple of tag synonym suggestions and similarly merged and created synonyms for identification->identify-this-bike and aluminium->aluminum.