I don't think the requirements need to be harder, per se. Just that in getting them, it requires you have a decent breadth of knowledge or experience with the subject.
At least with combat mentor, you have to do the lvl 80 role quests. While you can buy skips, that still means doing class quests and doing the shb questlines. And getting comms means spending time in combat with people you are matched with in DF.
Perhaps make the requirements be something like a collection of achievements. One could be the pentameld achievement -- or a new achievement that actually requires a single overmeld, since there are pieces out there with 5 slots.
Another could be earning a certain amount of white scrip -- this means you can't just craft 300 collectibles at 1 collectability rating.
And having master and folklore books. If nothing else, at least one.
Requirements like those would not demonstrate you being a social or helpful person the way comms might, but at least they would require you have a more robust knowledge of crafting and gathering than just how to level.