That does look fun, good luck.
I live just a few kms from the route in Helsinki, might be there to cheer.
Nice that the route avoids most of the busy Via Baltica/E67. The smaller roads are mostly very quiet. Looks like there's some gravel roads too, they're usually pretty easy there but very slow and soft if it's raining.
In Finland, the part from Helsinki to Lahti is pretty easy-going and fast, if not the most interesting. North from that, the road next to the lake Päijänne is pretty nice curvy quiet road. The drinking water to most of the southern Finland comes from that big lake, through a long tunnel. Then between Jyväskylä and Oulu the roure is on the road number 4 or E75 isn't what I'd choose to ride, though it's probably fine that far from the south coast that has the most traffic. After Oulu there's a nice smaller road, not much services or anything there, and then after Rovaniemi on the road 4 again, but that far up, it's pretty much the only road in that direction and rather quiet as there's only a few small towns after that. From there on the scenery changes, less trees, fells, reindeer, Lapland. And mountains after you cross the border to Norway.
In Slovakia that route passes just by the start of the climb to where there was a CP in TCR in 2017 near Poprad. Proper mountain scenery.
The area around the border of Poland and Lithuania is nice, small winding roads. In the middle of hilly fields, hope there's a tailwind.