I think it starts with the player first, then the quality of the guitar itself, then the pickups and then the amp.
A low quality guitar will never sound good in my experience even not with the most expensive pickups. What is not in there in the first place can not come out of any pickup. Pickups can contribute to the sound but only to some extent. I never had a pickup swap change my opinion on a guitar that I didn't like in the first place.
Also if you use a lot a gain pickups matter less. Maybe only for signal/noise ratio. But with (very) high gain you won't hear much sound difference no matter what guitar you plug in. This also counts if you use (too) many effects. When you play clean the sound character of the guitar itself and the pickups will become more apparent.
haa yes of course.. the player is a significant contributor..
many times I've seen guys go through repeated bouts of GAS in search of "the holy tone" only to find that once the euphoria of having something new has worn off they're back to being unsatisfied.. and so GAS kicks in once more..
in most of these cases that I've seen up close the issue was more that of technique than that of hardware..
so rather than trying to buy your way out of trouble.. invest in time and effort developing better technique rather than shelling out $$ on new things..
I consider build quality and pup to be on a par when it comes to the voicing of a guitar..
a bad neck joint for example or poor jointing between the neck and fingerboard can spill off all kinds of frequencies as well as sustain.. I have had personal experience of both of these phenomena.. once rectified the difference in tone was startling.. more so than I'd ever have expected in a massive way..
pup's are deliberately voiced to boost some freqs more strongly than others
once you have your well constructed guitar, your pup choice will head for the voice you're looking for
I agree with your point about gain.. a thing to add to that with clean tones is compression..
even with a clean tone, if you compress the tits out of the guitar it'll start to lose it's voice..
you'll begin to hear more compression and less of the guitar's voice
when I was working with Bulldog to develop my pups I made a bit of a discovery that surprised me..
very high gain pups with ceramic magnets riff like a demon - no surprises there.. lmao
but when you head north up the neck for a solo they can get pretty thin and shill [no matter how much gain you use]..
when we swapped the magnet out for an AlNiCo 4, the soloing tone was sweet but the riffing lacked focus..
for my tastes, the AlNiCo 5 was the 'Goldilocks zone'..
although it didn't riff quite so well as the ceramic, or solo quite as prettily as the AlNiCo 4, it was very good at everything and so the most flexible..
when I was working with Bulldog on my pups it was really quite a revelation..
it really is quite an art..