Data prices vary wildly by country. And no, you cannot use a foreign carrier to get cheap data and roam all year, they'll find you and charge out-of-package pricing for every gigabyte you've used (which isn't the standard rate).
If you can settle for 50Mbps (not really 5G, but already Aldi and Netto had a 100Mbps limit which is... basically the ITU definition of 4G), here's 50GB/month for 13 EUR: https://www.lebara.de/de/vertrag/hello-flex/hello-25-flex.ht...
If you can settle for 50Mbps (not really 5G, but already Aldi and Netto had a 100Mbps limit which is... basically the ITU definition of 4G), here's 50GB/month for 13 EUR: https://www.lebara.de/de/vertrag/hello-flex/hello-25-flex.ht...