I think the goal should be to get Debian to install on the phone without any patches, just like it would install on any old desktop computer. Once that succeeds all doors would open!
Being able to install Sailfish would be my next goal on the L5. Sailfish has Android emulation and does it pretty well (you have t to pay but its worth it). Also Sailfish OS it a lot better than what i tried with Ubuntu Touch, or Volla OS, or other OS’s that also have a Android support running, and Sailfish OS supports being installed on many phones already.
Third would be to run containers on PureOS10 that emulate environments and allow running apps native to say Sailfish OS, or Android, or Windows or any other OS making PureOS10 sortof OS agnostic when it comes to apps (this happens already with Flatpak, but could be expanded).