The only actual hard part was just that the rental market in Berlin has vastly more demand than supply.
* hopefully next month I pass a B1 exam, which tells you how hard it has been for me to get fluent.
If you were in London, it's like you never left home!
I'm not sure about how London compares, but Berlin has rent controls so the queues for open house viewings around here can go all the way down the apartment staircase and along the street.
I don't think it is? I moved to Spain from other EU country the same way, basically bought the cheapest one-way plane ticket I could find, spent ~1 month here before deciding I wanted to live here, then got myself the local residence card one morning and that's about it. Everything else just worked by using my passport in the meantime.
Well, exactly. Some countries require/required registration and residence card. That did not exist in the UK when it was in the EU, you just showed your passport/ID card when you needed to prove your right to be there (basically once in a blue Moon). Even now EU residents don't have any physical documents.
The National Insurance number @pdpi mentioned is unrelated as everyone has one once they work and an appointment is not always required to get one, and you can actually start working before you get one.
If you work as an employee there is also usually nothing to do regarding tax.
Then you need a social security number exist is different than the NIE, you need empradonamiento, you need to register with the health service and you need to set up your tax if you're going to work here (or if you live there more than 180 days of the year)
But then even with appointment one only gets a temporary permit unless one already got a job offer. One gets the permanent card only after starting a business or buying a property or getting a work.
Also to open a permanent bank account one needs to have at least a temporary residence. Otherwise banks can only open a tourist account valid for few months.
"Permanent residence" is also again different and requires residence under those criteria for at least 5 years.
Here in Finland for example the process is actually no different than for a non-EU migrant (same amount of time taken for an unproblematic application, same amount of appointments). You are just much more likely to be accepted but in fact they do still reserve the right to reject people. And it is, probably unintentionally, much harder to exist in Finland as a non-resident as you can't have a bank account, can't use foreign phone numbers for most things and any phone you can get is very limited (can't call many numbers, etc). I couldn't even log into the local eBay for the first 6 months. All the Nordics I would guess are similar.
And people have contested in the comments to you that Spain is not actually so easy as you suggested...
I actually don't know any western country that is as easy to move to as the UK was pre-Brexit. I still think the UK is in fact one of the easier Western countries to move to, especially if you can't find moderately paid work
Countries with a national id system I would guess tend to be more difficult overall though. And the UK famously is not one of those.