So here is the solution I have taken.
First, the problem:
My intention was to have WhatsApp (WA) on my phone, but due to privacy concerns, I did not want it to have access to my usual data. To me the phone is a computer in my pocket, so there are a lot of information about myself on the device.
My Fairphone 2 is running Fairphone Open, so the Guhgel free version.
I used to use an old Smartphone with a separate SIM-card. Unfortunately, this device finally was about to die and to become unreliable. So it’s now waiting to be recycled some day.
One solution is mentioned above, I did not try that one.
I created a separate user just for use with WA.
As on every Linux device, this user is completely separate from any other user on the phone. So no access to data, not even the apps are available.
Additionally, I inserted the SIM-card of my old WA-phone (thanks Fairphone for realizing a dual-SIM phone).
The contacts of the old phone were copied to this user. Unfortunately, I forgot to save the WA history, so that’s gone now.
I registered this “sub-phone” with WA and everything works fine.
WA has no access to my data but still will send my locations from WLAN and mobile phone cell coverage to Facebook. I cannot help with that. I just disallowed the usage of mobile data in background, maybe this reduces the amount of tracking, but I do not believe so.
Additionally, I did not give this user the permission to do phone calls nor SMS (text) messages. The phone says that if I did the other user (and thus WA) would see my communication history which I did not want to.
Now the trouble started. I wanted to share a photo taken as normal user with WA. How could I get a file from one user to the other. Here I realized how close the fence was. Amaze did not allow access, not even showed there was another user’s directory; nor did Ghost Commander. So everything working as it should.
My first trial was to use the FTP server from Amaze. You can start an FTP server on your phone and access it through WLAN from any other computer. I am using this to transfer data to and from my computer when I have no cable at hand. It also works from one user to the other on the same phone but in my case, it only worked in one direction. As the process was too cumbersome I did not work on resolving this problem but rather tried a different approach.
I am also using a self-hosted cloud server (freedombox project) which runs synchthing. So I created a separate folder only for WA files which are synchronized to the WA-user and to the rest of my network. The WA user has no access to the other syncthing network. I think it’s funny to having to resort to a cloud solution to exchange data on the same hardware, but it works.
I’d rather had converted my contacts to using a Matrix implementation which is also hosted on my single-board server, but that was helpless.
Probably there are not many people with the same use case, but I thought I’d share it anyway. Anyway it is interesting to know you can create multiple users on your phone and thus share it with your family and friends without sharing the data. Even though this use case is probably also extremely rare.