Lessons & practice upgrade the player, not the bass.
As for "upgrades". It depends on the bass & it's particular deficiencies. There is no single "best" upgrade. Only what is best for a particular bass and that will vary.
First one is anything to do with tuning accuracy, stability, or intonation. Tuners, nuts, bridge assemblies, necks, & frets. Imo, a bass that can't get into and hold tune with good intonation up the neck is nearly worthless.
Then the stuff that effects weight and/or ergonomics & feel. Strings, tuners, bridge. Bodies and/or necks in extreme cases.
Then tonal items like strings, caps/pots, & pickups. In that order.
The one that I always do first is string rolling. Mostly due to my usual preference for flats in most (but not all) cases but also because I'm a Fender guy that can't stand Fender roundwounds so the stock rounds get removed right away. I like Fender 9050's on most Jazz Basses & GHS Precision Flats or LaBella760's on P Basses. For rounds, I like GHS Bass Boomers. I'm also trying out a set of GHS Pressurewounds on a Jazz & I also want to check a set of their Balanced Nickels on my Precision too.
Then change the tone capacitance to .1uf for the vintage roll off. If you aren't sure if you want to buy a new capacitor & if your original tone cap is a .047uf one. Wire another .047uf cap in parallel with it if you have one handy for a total capacitance of close enough to .1uf to see if it's your cuppa or not.
So far, I have not felt the need to change any tuners other than the ones on my CV60's Precision that started shedding metal shavings. Many stock pickups are pretty good with the rest at least decent. I always try my best to make the pickup that came with it work for my needs. I do plan on eventually replacing the non-Duncan Designed pups with noiseless ones in my Squiers though. Imo, Squier tone circuits blow hard too. More like off/on switches.
While not technically a mod. A thorough setup can be worth its weight in gold. Ensure that you have a fundamentally good setup first before proceeding to anything else.