What do you usually use and offer when people ask you for contact?

Ask your family/partner/friends to use signal, it is idiot proof enough for most people. You can provide your email and phone number for acquaintances who you don't speak with enough to justify installing signal.

I'm not in any student groups, but you can make a dummy Facebook account and only login using their website from behind a VPN.

Banking app I used has blocked me from app after few years of using it when they realized I have it from "unofficial" source - Aurora store.

Just use their website and carry a physical credit card in the back of your phone case.

What's your way of transport after having few beers in pub? Do you use taxi via calling it directly or use that weird Telegram taxi addon?

We literally just drive drunk here. Is public transit an option? Again, you can sandbox these apps or use their respective websites.

Is there a way to convert google map pins to open source solutions and vice versa? What's your recommended software for directions? What do you use for driving?

Organic maps is good if you have an address and only need directions. If you need complete records of where local businesses are and traffic info, google maps is your only option. I'd set it up in a sandboxes container though, at the very least.

Do you track fitness activities?

No, and I'd argue that nobody other than serious athletes need to. That being said, you can get a tracker that does not connect to your phone or the internet.

Do you have good phone recommendations? I know that GrapheneOS+Pixel is one, but what about others?

Graphene+pixel is head and shoulders the best option, especially in places like the US where you have to worry about illegal searches. Privacy requires security. That being said, there are alternatives if you cannot afford a new device: https://eylenburg.github.io/android_comparison.htm