Thanks all for the ideas and suggestions. Some quick feedback:

- What's the difference with Moto Z + Keyboard with this device?

At least 7mm thinner
Much better feelings. Top thinner + Bottom thicker instead of other way round.

The advantage of Moto Z is you can take off the keyboard and change to other mods. But I suppose there will be a group only care about the keyboard (like me). So the "new N950" is a better option.

- Regarding Lineage OS, this is in my opinion the best one for Android. Lineage OS is free and open source, but can I ship as an default OS? I couldn't even find anyone to ask and contact. Any ideas on that?

- Regarding Sailfish OS, the barrier is mainly front cost and time. This is not something I can achieve easily and instantly. It's much easier said than done.