After receiving my new phone the best handheld gnu/linux machine for 2019 seems to be Sony Xperia XA2 Plus with Sailfish X.

- linux/gnu base - check
- Terminal - check
- Easy to setup ssh access - check
- Full control of the device - check
- 2 sim cards, SDCard option, enough memory (6GB) - check
- Everything on the device open source - does not pass

Negatives:
- This is par-course for Linux devices so I'm not sure if it warrants mentioning, but it is of course half-baked and expect some things not to work. As always with linux based machines, do your homework to find out if something essential to you is not working.
- It's a bit big. I have to use it a bit longer until I arrive at final conclusion. Usually I like more compact handhelds, but even though I was shocked at the size at first, it hasn't been hindrance so far. Quite the contrary considering I got to replace two devices now that I can fit 2 sim cards in it.