Me, I'm a geezer with a geezer's viewpoint. I say fix your core, not your bike. Get PT, join a gym, stretch every morning, all of the above, whatever it takes. Human bodies are mostly fixable. My history says that "If it hurts, work it" is a good motto. Besides getting a long happy life, it's cheaper.

OTOH there's a possibility that your current bike is a full race machine with a very large saddle to bar drop, in which case anything will be an improvement for casual riding. IME anything with a saddle to bar drop of less than 10 cm will work fine for very long rides. In my mid-70s, I went from a -17° to a -11° stem, still slammed, though that wasn't really necessary. I've been a gym member since '79.

I'm not a big fan of stem/bar setups which don't allow easy component changes. I know that's getting popular, but IMO it's a mistake. I'm riding a 2000 carbon bike which is still perfect for my long rides in the mountains, partly or maybe mostly because it's been easy to change out components.