Have a look at https://www.Odoo.com

– support German CoA (SKR03 & 4), and Taxes
– automate payment via SEPA
– sync with 24.000 banks across the world (I haven’t checked for germany, but a camt.053 module is under development and will be released soon)
– automate sending letters by mail or regular mail (payments follow-ups or invoice)
– has an API: https://www.odoo.com/documentation/9.0/api_integration.html

Odoo Accouting is free, unlimited users, on the cloud.