Best distro for my use case

[ryo]

1. Make and receive calls

Mine isn't being shipped yet, so I can't answer this one at this time.
But you can probably test it using a second phone, a fixed line, or maybe with Skype if you have a credits on it.

2. Send and receive SMS

Same reason as above, I can't answer yet.
Try sending an email to your mobile email address from a PC, and then again from another phone to either your phone number or mobile email address (whichever is the easiest, they're both specific to the same USIM card anyway).

3. Use Signal messenger

I'm not using Signal, but considering what others have said, you'll probably need to use Anbox for that as a workaround.
Or otherwise install an Android OS for the PinePhone (which probably kills one of the main reasons we all want to have a PinePhone in the first place).

4. Use email (IMAP only)

Any mail client with IMAP support should work.
However, if you're using Gmail, you might need to use Anbox or install the desktop version of Thunderbird.
Google is very picky when it comes to what you're allowed to use as a mail client or not (for example, Evolution and KMail both don't work with Gmail).

5. Use Nextcloud for calendar and contacts

You can use Nextcloud through the browser, which works well enough even on small screens.
The Nextcloud team is pro-open source from what I understand, so it would surprise me if they'd not make a mobile app for Phosh or Plasma Mobile at some point.
Or otherwise you can make your own app, Nextcloud has its APIs fully available (which is why there are all those 3rd party Android Nextcloud apps available).