The official site is best to get started. Personally i find it the most easy to run a container and configure a secondary Firefox profile: https://geti2p.net/en/
The push notifications would be a issue for me. I am using GrapheneOS without any google services.
Also the calendar i am not 100% sure how I want to do it. I currently use Nextcloud and Caldav. Which for me works great when syncing with Etar on my phone and Evolution mail in the desktop. For my dad I have setup caldavsynchronizer for outlook as that is the email client he has used for years. When i would use Tuta id loose the nextcloud calendar because it can in no way synchronize with Tuta. With proton on the other hand I can use the bride for email and use the calendar how i am currently using it together with Etar on the phone.
On the other hand if say Tuta providers a calendar that is integrated and works with both the email client on the desktop and on the phone. the same goal is accomplished.
Not sure if it is still possible. But I believe you can on Tor if you spoof your user agent to be a black berry or some other device. Then you can grab the direct download link to the iso and download it over Tor. Been some time since I have done this though. Keep in mind this is from memory so it might be slightly different.
Only the pixels phones allow for relocking the bootloader with a custom ROM. Might be possible on a few other devices when jumping true a load of hoops and potentially bricking the device.