You should go with a caldav server. Personaly i prefere Baikal. You could get a overview on the awesome selfhosted site.

Android has a calendar store. An App like DAVx5 would sync with your caldav server and stores the data inside the calendar store of Android. Every calendar App would then work with this cause for the app it looks like a local calendar.

For the web i’ve found 2 projects, agendav and bloben. I’m not happy with both of them and go with thunderbird on the desktop.

For PSD2 exists a project named Hibiskus. Its a Desktop tool to manage your finances. It syncs over PSD2 with the bank.

There is also a server version of Hibiskus. It syncronises after a schedule with your bank and provide the data over a well documented REST-Api.

https://www.willuhn.de/products/hibiscus-server/

The project website only exists in german.

I’m running a self hosted combination of hibiscus and firefly III to fully manage my finances with automatic synchonisation.

1. Hibskus Server syncronise with my bank accounts
2. Node-Red query’s the REST Api and creates a csv file
3. the csv file is read by the firefly III importer

Not the simplest way, but fully self hosted. I really don’t want to provide my bank statements to a 3rd party