There's just now way it doesn't neck dive
Absolutely false. I have two Sandberg SLs. Neither neck dive and are totally comfortable. One is 6.5lbs, the other 6.1lbs.
I also have a G&L jb4 in Empress at 7.5lbs. It neck dives a very slight bit.
Sandberg's hardware is proprietary making it super lightweight.
My Sandbergs are the first ones I play. The g&l is the last. The g&l is an awesome bass, but I've had fret playability problems with it causing a full fret leveling to be done. It still doesn't play as well as the Sandberg.
There's nothing I can say bad about the Sandberg. The g&l is also a top line bass and if ordered is fully customizable, but it's more like a Ford, whereas the Sandberg is more like a Mercedes.
Note that the comment about the Sandberg being returnable is mostly only true in Europe. In the USA, they mostly must be ordered and returnability depends on the dealer.
About the dealer: know that Sandbergs aren't boutique per se, but all options are on the table, for a price. The dealer will guide you but you have to ask about specific things you want. And even, all options aren't specified on the website. This is also true for g&l. So the quality of the dealer is critical. I asked for a non reversed p pickup on my Sandberg California PJ and they were happy to do it, no charge. Normally the p is reversed.
Ask about what pickups you're getting. That part is confusing. I ended up changing my p to a fender original, which I love.
Both basses can easily go to $3k, but either can be had for well under $2k. My sense is that a basic Sandberg California p will end up being LESS than a g&l.
The fretwork on the Sandberg is better. One thing to look out for though is a high zero fret. They seem to all come that way.
The Sandberg will take at least 6 months to make unless you can find a stocking dealer.
edit: There are a few. And for a Sandberg precision type bass made as an sl, you'll probably have to order it.
I don't care about resale and at your age neither should you. The Sandberg is a unique bass that is designed to fit your needs much better than any other non custom boutique bass in the world.
Those are random thoughts. Feel free to ask questions.