Why do you automatically assume that if you get the Squier that it will require upgrades? Way too many people seem to think that just because it's an "inexpensive" bass that it's somehow lacking. Changing pickups will make it sound somewhat different (maybe) but better? Highly subjective. Different strings often have a much more dramatic change in tone than pickups do.
You really should give it a chance before you just assume different pickups will somehow be an improvement. Unless you know in advance exactly what improvement you're going for and exactly which pickup will do that a lot of folks spend a lot of money chasing their tails looking for some magical sound they can't define in their own minds and thus have a difficult time ever finding to their satisfaction.
Bridges - the Fender-style bent metal bridge has been the standard for 70+ years and it works quite well, obviously. Unlikely you'll see much improvement (if any) in sustain etc going to a high mass type bridge, and depending on the Squier model it may well come with a high mass anyway. Undoubtedly some will come along here and argue that point, however anything gained there will be incremental at best.
Tuners I can see a bit more. Generally stock Fender/Squier tuners will give great service life and hold tune just fine. Going to a finer gear ratio tuner will make things a bit easier to get just exactly right but again the gain IME is small and incremental at best. Where is your Point of Diminishing Returns investment wise?
I play large places/gigs rotating between two bone stock Squier P's (a Matt Freeman and a Vintage Modified PJ) and a Fender P I paid 5X more for and tonally/looks wise/playability wise no one knows the difference, myself included. These days the Squiers, Epiphones, Sires etc of the world are perfectly good working mans instruments in stock form and just because they're "inexpensive" doesn't somehow necessarily mean they're "cheap". Give it a chance before you just start throwing money at one. Or just save up and get the one you think you want and save yourself the headaches and frustration of chasing some poorly defined thing you think you hear in your head but somehow just can't quite find.