Hi Alex,
Yes, it can look overwhelming. If you are not sure then the best choice is the entry that is already highlighted when you enter the page for the first time. That is the default chosen by the IPFire developers as the best security option. In most cases you don’t need to change anything on the Advanced Options tab unless there is some other limitation.
In your case your android phone doesn’t allow IKEv2 which is the default Key Exchange selected by default.
As a good first starting point you can use the default options highlighted when you select IKEv1 and see how that works.
In terms of potential to be able to use IKEv2 on your android phone, the Strongswan app would probably make a good option as it is the android version of what is used by IPFire for it’s IPSec server software.
The only thing I am not sure about is if it works with Android 9 or not but if you try and install it, there should be some message if it is incompatible with your android version.
Good luck with your setup and if you need any more help there are definitely people on the forum who are using IPSec and will be able to help.
EDIT:
Just checked the app specs and it works with Android 5.0 upwards so should work with your phone and will give you the secure IKEv2. I would recommend using that app rather than the insecure Android 9.0 internal IKEv1 option.