I managed to install most important apps when I got my FP6 2 months ago, evem though I noticed quite a few apps weren’t supported by Murena.
However, now my phone won’t install a single new app - once you hit install, nothing happens and you simply have just the option to cancel.
That seems to be a common issue in their store (saw it here and reddit before) . Maybe you can try Aurora Store instead. I think reinstalling their store was also a solution but can’t confirm.
Thanks! Is it this one? Auroraoss.com
Yes, I’m using that one lately (on Android though) and it’s quite good and shows also privacy concerns.
removing default system apps is not possible
(even disable is possible using adb shell)
update is only possible along the system update
@piero Of course you can.
I recommend you @Jon2 to go through factory reset on your mobile first. If after this it still does not work, uninstall and install again the desire default app.
Anyhow you can go through @Ignacio chose as well 
The Aurora store is available through the F-Droid store. As many people often have trouble with the App lounge several people advise to use Aurora and F-Droid instead.
I also found this interesting, since I noticed there are several users complaining about the updates and the conflict with MicroG and GooglePlayServices. There are as well some apps which are not working properly in AppLounge from e/OS ( people are reporting it at Murena Service Support).