€250k @0.4% = €1,000 a year. So it would take 3.5 years.
So go for the cash back as it's possible you will have switched within 3.5 years.
Yes, your figure of 3.5 years is correct. I get the same answer when I updated my spreadsheet to subtract 3.5k from cumulative interest of EBS.
The principle of €250k is decreasing all the time. What am I missing here?