I don't know that it's the replacement parts business so much as concern about parts builds competing with their stupidly overpriced high end models. Let's face it,the reason people like us don't buy those high margin basses is exactly what you mentioned about the diminishing returns of non-electronics upgrades. The guy who buys a $2000 - $3000 Fender or Gibson isn't considering a DIY Build. Unless it gets cheap enough for small luthier shops to compete on finished instrument prices - and it isn't close - those parts aren't their competition.

I saw some specific numbers for Warmoth's P-Bass body vs USACG's Pea Bass discussed on the TalkBass forum. A guitarist friend of mine we who has bought Strat parts from both said it was the same for guitars. Of course both could be full of it and I wouldn't know better without more research than I've ever bothered to do.

I'll say this for the 2 SX basses I've played - mine and my daughter's Ursa Jr P-Bass. The pickups are much better than you get on most ultra cheap models. They're certainly a big step up from what you get in any Squier. In fact, every account I've read about people swapping the Jazz pickups out for standard Fender MIA versions concludes the difference is in character,  rather than quality. If you prefer some of the third party alternatives, that's obviously a different story.

I do plan to swap out the tuners and bridge eventually, although they're good enough that it's not a high priority. However, there is a tiny bit of neck dive which appropriate upgrades should take care of. What it does need badly is new pots/jack/wiring and proper cavity grounding.

The one thing that did shock me when I first picked it up was the truss rod. SX basses are notorious for needing extensive setup, but the truss rod on mine had to be tightened almost 2 1/2 turns. It could have been worse, though. At least you don't have to take the neck off to reach the truss rod nut, like on some cheap instruments.

In any case, for a little over $200 shipped, and including a generic,  but perfectly adequate, hard case, I have no complaints.

I was also fortunate to find a nice deal on a decent amp. For less than $250 I got an 80's Trace Elliot AH150 SMC GP7 and a 4x10 Hartke aluminum cone driver. A lot of people don't care for the TE sound, but I find the mid boosting Pre Shape to be nice for rounding out the J-Bass sound. The cab is an old Transporter, so not exactly what I'd prefer, but good enough to get me through until I can upgrade.

I'd be happy if I could find what I want for relatively cheap, whether it's off the shelf or as a kit. Unfortunately, if you 32-inch scale basses are fairly rare to begin with.  Make it a lefty J-bass witha P-bass nut width and you might as well be looking for Bigfoot hanging out with the Loch Ness Monster.

The sad thing is it's my own damn fault I have this problem to begin with. Although I am technically left handed, I  actually have a high degree of cross dominance. That means I can generally learn things either left or right handed with equal ease. Also, I had already been playing cello for 7 years before I was given a right handed bass.  Being young and foolish - and also a fan of both Paul McCartney and Jimi Hendrix, I chose to have it restrung and play lefty instead. Whenever I'm shopping for a bass I find myself wanting to go back in time and kick my ass for that.