I think it’d be a lot easier to stick to software initially.
Why make a SAFE phone when you can achieve a high level of privacy through SAFE apps, and have a huge user base?
If you want deeper level software, create modified Android firmware with SAFE baked in & privacy tweaks that is compatible with a number of phones (similar to cyanogenmod etc).
Then, if hardware is truly a limiting factor, seek to partner with existing phone manufacturers to create ‘sefe edition’ phones, but the history of niche phone projects, even with partnerships is a graveyard with no notable successes I can think of.