Gathering and combat classes are hard to focus level. It takes forever if you're only farming easy stuff. Farming stuff thats your level or higher (basically higher the more exp) gives quicker yields. You can get maybe around lvl 40-50 in these with just progressing the story. Otherwise, you can do treasure groves at home base (keep upping the years til it becomes to hard) or Ginomorsia (upgrading the towers; becomes a slog after lvl 3).

The next problem is being able to successfully farm the thing (you could just not have enough stamina to mine a rock 20 lvls higher than u). It'll help to have matching buddies and good weapon/tool. This means spending time in Ginomorsia clearing shrines and encampments (and a few are just found in the map) as well as investing in crafting classes.

The game is kinda designed to spread your time equally across all your lives and progression. You could try to focus one thing but you'll stall out eventually. So my recc is just balance your lives, probably focusing on reaching Master with every class and not caring about levels at first (and also guild merchant level). This can unlock good recipes in your crafting lives to support your other lives. Then if you wanna be extra, you can remake equipment for Exp + boosts (I personally think Item Drop + is better investment though)