Hopeful

You're right that there's no vendor out there selling a really performant device officially with a close-to-mainline kernel.

But that does not mean that the situation is all bleak. Firstly, many older Qualcomm-SoC devices (OnePlus 6(T), Xiaomi PocoF1, Pixel 3a) are increasingly well-supported in distributions like Mobian or postmarketOS (yes, camera is a pain point, but that's not going away soon, as the whole “how do we properly process the stuff sensors put out” stack on “desktop Linux” is still WIP). If you insist that you want to buy a device new from a vendor that's at least not against mainline Linux support, you have some options, too:

The Fairphone 5 and the upcoming SHIFTphone 8 both are in that realm, as developers employed by these companies are involved in the mainline-porting of these devices. Now, both can't be used as phones with postmarketOS yet (notably, the Fairphone 5 lacks audio support) but work is ongoing, so there's reason for hope.

So, it's not all bad, even without a next-gen Librem 5 or PinePhone successor. And if you're open to staying with a vendor kernel but pairing that a GNU/Linux userland through libhybris/halium, there are even more options: FuriLabs FLX1, various Volla Phones, Jolla C2 and a variety of XPERIA 10s supported by Sailfish X.