rooted phone su user changes Is there a way, once rooting is done, and i have customized my phone a bit (a change in one of the config files of linux), to change the user accounts and permissions? To create a new user account and change the user that is used by the phone to be a non-root, non-sudo, standard user. And all apps to use this or another standard user account.

And my app (that i installed after rooting) use another account. Only this account will have permission to read and write my app's directory.

Finally change the su user password, so that others cannot get into it unless they have that password.

So in effect protect my apps directory and allow the phone to work with a non sudo user from then on?

Next time phone re boots it uses another user say A (non sudo); without access to my apps directory. And when my app runs it uses user B (also non sudo) which has access to its dir. Others cannot read or list files in it or change permissions. So in effect my apps directory cannot be read by the user of the phone, in this new set up. I understand there will be ways around this. 

But is this possible and how? 

Rooted using https://forum.xda-developers.com/android/general/root-samsung-galaxy-on5-t3435457 but i can do systemless root if that is the way.

Phone model : Samsung On 5 Pro SM-G5550FY. Thank you much.
On Android stock from Lollipop/Marshmallow there is the multiuser. You can create a guest account completely divided from the owner. I don't know if this function is present in touchwiz, search it in settings.