Hey Sven,
I am planning a similar tour from Lübeck-Travemünde eastwards around the Baltic Sea. Rough planing using EuroVelo 10 as a "guideline" for the rough planning, Kaliningrad and St. Petersburg being only options. Information:
https://keeponmoving.plic.de/baltic-sea-round-tour-2017/Average monthly mileage: 2,500 km
Cheers,
Patrick