I don't quite understand the proposal, and much less the intent. Do you want to make gaining levels slower? What would be the gameplay implications, and why would they be good?
#Increase the exp-value curve between tiers for fishing
devout ermine
lunar dagger
this seems unneeded to me. I'm a generalist and my fishing hasn't really been lagging behind anything so an xp buff just seems unnecessary. Unless it's just the exponentiality you want, like each tier gives x3 and levels cost x3 to encourage people to fish at their highest tier. Which would be a bigger conversation