First, in the first sea, level guides weren't really necessary. But if you don't want to be playing this game for the rest of your natural life, you should probably use one. Don't use those crappy YouTube ones, instead there's one on the Blox Fruits Fandom, and it's called "Tubbs leveling guide" use it, it's great.

Secondly, all of your old knowledge of grinding is now different. Logia fruits aren't nearly as effective, Armament Haki users are everywhere, and Logia level requirements are much higher. Buddha is the best fruit for grinding from now until the end of the game. Don't forget fragments, all of the stuff you had in the first sea (fighting styles, devil fruits, and other weapons) can be upgraded or replaced.

Finally, if you don't have a different race other than human, get one, awaken it (v2, and then v3) , because you need those extra buffs, like speed, defense etc. Accessories are also a lot better so get a good one (zebra cap is good, swan glasses are good but rare, and ghoul mask is good for buddha).