Sweet! Let me know when you are nearing Estonia. Basically your only difficulty here would be the Russian visa.

Between Calais and the Russian border, the only stretch of road you need to be even remotely worried about is Poland, just because their freeways don't extend quite to their borders, and you end up jostling for road space with lorries. Unless you are planning to see some of the better Polish cities (Krakow etc), consider taking an overnight ferry from northeast Germany to Latvia or Lithuania. Poland as a transit country is mostly just big and annoying.

You can set a Parking Available flag when searching for hostels on booking.com. Most Polish transit motels have secure parking.

Good plan.

I would say, skip Moscow. Go up via the Baltics into SPb. Then aim for Norway, which you can do in a couple of ways. Either ride across the southern coast of Finland and take a ferry from Helsinki/Turku to Stockholm (well worth a stopover, unlike Helsinki), or up to Vaasa and the ferry to Umea, or just ride north through Russia and cross over near Kirkenes (more adventure, crappier roads). Then head over to the Norwegian coast and follow more or less Route 67 down. Different options to cross over from Oslo/Gothenburg/Malmo.

Luggage wise: the panniers will be fine, I suggest a topbox as well - keep it empty and put your helmet and gloves in there while you're walking off the bike. If this solution is insufficient, you will know this by the time you hit Germany, at which point you can always find a Louis Megastore and get a cheap drybag for the pillion.