I didn't even really use leves for fishing outside of opportunity. Just went and explored every fishing location with Field Manuals, then went on to fish the Tiny Tatsunoko and farmed some Dusklight Aethersand (I mean, I was going to farm it anyway so I might as well get some xp while doing it). Made a few millions selling leve and GC turn-in fish, too. Outside of 68 to 69, it was pretty painless. I guess that's not the most efficient way but to me fishing is all about exploring and filling your log anyway.
But yeah, don't use leves on Miner and Botanist (for the 60 to 70 grind anyway). Farm collectables instead, it'll buy you every single Tome of Folklore before you even reach 70.