It doesn't matter where your savings are, but I'd suggest you keep it as simple as possible, to make it easier to assess your application.
Imagine you're in underwriting in the bank. You've 2 applications on your desk. Application 1 has a current account and a savings account to look through.
Application 2 has statements from 4 or 5 different accounts with different banks, and a few brokers for investments.
Which one would you choose to look at first?
Large regular transfers to gambling would be red flag.
Degiro will be fine so long as there's an investment to show for it.