German broker (Flatex) local tax implications for UK tax resident

[BigPrize]

I read with interest that Flatex is suggested as a viable 'low cost' alternative as an offshore account for UK residents. As a non-German resident, what sort of German taxation would you be subject to (and therefore may need to rely on DTAs to recover)? I am particularly interested in trading, though mainly holding longer term, German-listed Irish ETFs and, potentially, US Treasury Bonds. Would any dividend or bond coupon be subject to WHT in Germany?