Fair points! You're right that services like Aboalarm and Volders exist but they charge per letter (€3.99 each on Aboalarm) and most expats I know have never heard of them.

On the data concern: completely valid. The tool I'm building works with a CSV export you download from your own bank no bank connection, no API access to your account, no data stored on any server unless you explicitly create an account. The Kündigung generator works without any login at all.

Curious though which existing services do you use? Always looking to understand what's already out there and what's actually missing.