I just installed the BVG app and registered and noticed one minor potential problem: for country it only has options for EU countries. It let me register just fine when I just put in my address but put in Germany for country, but will it cause any kind of problem later in use?

TXL bus route change
 

Achtung! Effective this afternoon "until further notice", the TXL bus no longer runs to Alexanderplatz. They seem to be terminating it at Brandenburg Gate, in that the announcements (in German only that I have seen) for each of the destinations between there and Alexanderplatz all suggest taking the 100 bus to Brandenburg Gate.

If you have to change anyway, it is likely faster now to take the two connections route via U and S bahn to pick up the TXL a few stops away from the airport. This info has been posted here in FT recently.

I checked because I am currently in Alexanderplatz and was planning to take TXL on Saturday.

TXL bus on normal route again
 

In fact I am on it, picked up at Alexanderplatz. There is much construction between there and Brandenburg Gate so maybe this was short term to keep to schedule?

Do they have Uber in Berlin?

No.
Taxis are cheap and we also have very good public transport.
If you are intent on using app-based taxi then download My Taxi.

Yes, they do. Just used an Uber last week to get from the Farbfernseher club back to my hotel room.

I second this. Order the MyTaxi when you are leaving the aircraft, he'll be in front of your exit gate within two minutes. No major city airport can match that.

This would have been the "not-even-semi-legal-Uber-car-hire-service".

The municipality of Berlin who control taxi licences here have banned the regular Uber minicab service.

Not relevant where you are going from/to.

We will get the 7 day transit pass so this would make sense. Do they let you haul large suitcases on the bus?

I don't know yet where we will be staying, but am drawn to Keuzberg (61) where I used to live for a few years in the 80s.

Dude, despite living in Berlin, you are terribly ill-informed. Uber isn't illegal in Germany. The company is fairly active in Berlin and Munich. Not in other German cities, just in those two. What you are writing about Uber is BS.

You can use Uber in Munich and Berlin (only UberX and Uber Black in Munich). At the moment there are pending lawsuits. Uber Black is subject to a big lawsuit at the moment, the sentencing will be at 13th December.
They now also offer UberTaxi in Berlin which is in fact a regular Taxi (also same tarif).

Most big companies have pending lawsuits (Deutsche Bank, Bayer, you name it)...

Uber is about to re-start service in Frankfurt. They will offer both Uber X and Uber Taxi.

I kow, but this one is quite essential...….