A cash flow is essential to sustain a Linux mobile ecosystem. And in the Linux world the sell goes often once. Just when selling the device itself. But software has to be maintained, and people have to be paid. Therefore, I highly recommend to buy a native Linux hardware device, independent on Android, if you want to support mobile Linux, and have the financial means.
There are options to donate to different mobile Linux projects as well though. Only a small donation can already make a difference, especially if many people are providing such support.
It is hopeful to see how different Linux phone software projects, and Linux phone competitors in a way are all working together to further a truly free operating system. The phone market is large enough for native Linux phones to exist next to each other. So not Linux as an aftermarket, but Linux native phones that support the whole mobile Linux world. And yes, also there is place for aftermarket Linux support, but I doubt if this would be enough in the long run.
If you look through the list of recommended devices for Nura (formerly postmarketOS), the only one that has full software support for all the hardware is the Librem 5, and the second best supported phone is the PinePhone.
The rest of the phones are dated Android devices which all have significant functions that don't work correctly. The processor with the best support is the Snapdragon 845 from 2018 in the OnePlus 6/6T, POCO F1 and SHIFT6mq, and all of those devices have issues that a geek like me can accept (like downloading code from github and manually compiling to get a partially functional camera), but it isn’t really an option for ordinary people.