I've been trying to get to the highest baking level at Fantine's Bakery but I can't find the achievement for it. I assumed there would a tiered achievement for Fantine's Bakery. I can't seem to find it, though, and don't know what category it would be under. I might be overlooking it.

Where are the Fantine Bakery Achievements located/do they exist?

Sorry they don't exist. There are several suggestions about it on the suggestion board, achievement sub board. You could go lend your support there. :).

Thank you! Super weird there's not one...

it has been talked about, by staff even, but nothing has come of it yet.

Is there a highest level of the Baking with Fantine game? I presumed it didn't have an upper limit, because my baking skill is higher than what's needed for the "most difficult" ones by five or six levels.

oh I have no idea, I just wanted to be able to make the 'most difficult' items

it's likely that the needed level will go up by at least one when new recipes are introduced. This has been true in the past.