I’ve never seen a roadmap

Neither have I, sounds like a good project in itself if it doesn't exist. 'Drivers for xyz, reverse engineering something' is part of the problem, phones usually (nearly always best I understand) have proprietary blobs of firmware to a greater or lesser degree and it's a moving target different between manufacturers and most often models. Qualcomm modems are particularly egregious for patent reasons. US trade deal enforced global DMCA laws make reverse engineering legally tricky. Hence the desire for linux specific hardware platforms.