How much theoretical knowledge does playing the Berlin Defense require? Sorry if this question is opinion based, however I think it can be meaningfully discussed.
I am researching the Berlin Defense and am reading contradictory opinions on how much concrete knowledge is required to play it.
Some people claim that

It requires very little knowledge of variations and one can rely almost
entirely on endgame principles
Almost all sensible moves are playable
The game is slow and of positional nature, often drawish

The other side claims

It is one of the most theoretically extensive openings
Plans are often not obvious, but one needs to know what to do in order to not fall into a hopeless position with no improvement possible
Play becomes sharp and concrete, requires knowledge of variations, the better prepared player wins

Where do the differences of opinion come from?
Back in the days of over-the-board chess I use to run themed blitz tournaments over the summer to keep members of our chess club occupied in the off season. In July 2019 I ran one on the Berlin Defense. Only 6 players turned up instead of the usual dozen or so (surely not put off by the boring reputation of the Berlin?) so I ran a 6 player double round robin giving each of us 10 games in the evening. That was the most hard work I'd done for a long time and it had nothing to do with playing in and running the tournament (which I did most weeks) and everything to do with the slippery nature of the Berlin. Every single game was a real struggle.
To help players get in the mood and prepare I usually sent out an email with links to a couple of YouTube videos on that week's opening. That week this video from Yasser Seirawan was one of them. I'd strongly recommend you watch it. I will say that as prep for a one-off game it is not suitable. You will either be very frustrated at the pace and apparent lack of a destination or Yasser's soothing voice will put you to sleep.
However if you are going to make the Berlin your goto defence against the Ruy then it will give you your first and most important idea what the opening is about.
When you are learning a new opening quickly the most important thing is to understand what white's plan is and what black's plan is in the opening. That way you know what you are trying to do and what your opponent is trying to do that you have to stop.
The second most important thing when you are learning a new opening is, as well as knowing the plans, you have to be comfortable in the kinds of positions you will get in the opening. By the way, that's why Kramnik beat the mighty Kasparov to become world champion. Kasparov was deeply uncomfortable in the Berlin positions and Kramnik wasn't.
But in the slippery Berlin nothing is clear as Seirawan demonstrates in his video. What are white's plans? It should be fairly obvious. Here is the starting tabiya.
[fen &quot;r1bk1b1r/ppp2ppp/2p5/4Pn2/8/5N2/PPP2PPP/RNB2RK1 w - - 0 1&quot;]

White has the advantage of a viable kingside pawn majority which should be capable of creating a passed pawn. Black's doubled pawns on the queenside make it relatively easy for white to prevent a black passed pawn there. On the other hand black has the two bishops and the e pawn is really one square too far forward. Life would be easier with the pawn on e4 instead of e5.
One important middle game principle is that any given pawn formation will have an optimal number of pieces. A very cramped black pawn structure will be terrible for black if both sides have all their pieces but perfectly fine for black if several pieces have already been exchanged. Similarly far advanced white pawns are good for white, white controls more space, if all the pieces are on but the position becomes over extended if too many pieces have come off. The e pawn is just a bit too far forward given how few pieces are left. It can very easily become a problem child.
Nevertheless, white's plan is simple and obvious.

Arrange the queenside pawns so that black can never create a passed pawn there.
Advance the kingside pawn majority to create a passed on the kingside
Queen the pawn.

Well, that was simple and obvious to state but the next step is working out which moves to make to put the plan into action and that is where the problems start. And that is where Seirawan's video is so good because it slowly, frustratingly makes it clear just how difficult that is. Lots of plans have been tried over the years and none of them really quite work unless black doesn't know how to counter them.
OK, so white's plan was easy to state but seems very difficult to put into action, but what on earth is black's plan?
Well this is where it gets even more difficult and even more hard work. Black's plan is just to counter and frustrate whatever white tries to do. Again Seirawan's video will help make this much clearer.
OK, let's look at some of the propositions.

It requires very little knowledge of variations and one can rely almost entirely on endgame principles

Certainly not! If your opponent knows what they are doing and you don't then you will lose like the slowly boiled frog without ever knowing just what happened.

Almost all sensible moves are playable

True in the starting position of the tabiya. There are no proven plans for white. Black's plan is just to counter what white does. The player who struggles and fights the best will win. Because of the unbalanced nature of the starting position there is no obvious route to a draw for either player. You just have to fight.

The game is slow and of positional nature, often drawish

Well, yes. It is slow and positional to start with but the thing about slow, positional, drawish games is that one slip and they can become tactical and lost very quickly. In the absence of a clear plan for your side you must always be countering your opponent's plans and that is a big weakness for weaker players.

It is one of the most theoretically extensive openings

If you are black then you certainly need to know and be able to counter all the current white plans. If you are white then you need to know what to do if black doesn't correctly counter your plan. That may involve a quick transposition into one of the other plans. You also need to be very well prepared if you are going to try and avoid the draw.

Plans are often not obvious, but one needs to know what to do in order to not fall into a hopeless position with no improvement possible

Exactly so.

Play becomes sharp and concrete, requires knowledge of variations, the better prepared player wins

Again, this is absolutely correct, although it can take some time to reach this stage. Your best way of not being on the losing side is to know the starting plans.
The only real contention is the first claim that knowledge of variations isn't necessary. Perhaps this confusion arose because the variations that arise are generally not tactically sharp variations, at least in the first instance, where one side gets quickly mated or loses material. Instead the variations lead to positionally lost games. The result of not knowing these variations is still the same as not knowing the sharper variations which arise in other openings.