iPhone keyboard writes wrong words and numbers, misspells things, or doesn't react at all

I have an iPhone xs. I received it with a minor scratch on the display. In the beginning it worked wonderfully.

But in the past two weeks it started to behave ridiculously.

When I type certain letters, such as the letter G, the keyboard would write the neighboring letters F or H, nothing at all, or gibberish words and numbers that do not make any sense. Usually, after the misspelled 'word' the iPhone sets a 'dot'.

For some days now, it also has become a nightmare to swipe on the iphone. It is works okayish when I swipe to unlock the iPhone but inside some apps one swipe takes me several minutes to perform.

I bought this iPhone from a friend only one year ago. It used to be in very good condition.

To troubleshoot the problem, I

1. cleaned the display (no change)
2. deinstalled and reinstalled apps using the swiping function (no change)
3. turned off, turned on the iPhone (no change)
4. removed the protection display layer (no change)
5. erased all data from the iPhone and did a full reset without any back-up data (no change)

When I had reset the iPhone and I was just asked to type in my wifi password and icloud details, the keyword went nuts again and it took me many minutes just to type in the letters and numbers.

Of course, it looks like a hardware thing now. So, I would like to ask you for your experience whether it is even worth it financially considering fixing it.

If I send it to Apple or another repairing store, how much euro would it cost more or less, preferably in Germany? Do you perhaps know since this an issue that other iPhone users have faced in the past that always has the very same root cause or could it be anything that triggers this keyboard syndrome?

Thank you very much for your insights and help. :)

Ida