Hello!
I would like to share the route I made for myself and my story how I finished it recently. The route I will describe starts in Belgrade, goes along the Danube River, through Djerdap National Park to the border with Serbia, and returns to Belgrade through Kucaj-Beljanica National Park. The route is compilation from parts of different EuroVel routes.
Longs story short, I'm using the cheapest Decathlon Triban 100 (~300 Eur) with replaced transmission. Instead of stock 7 speed one I installed Shimano Sora 34/50 & 11-34 (bought it on Aliexpress for 200 Eur). Also instead of stock tires I installed Continental Contact Plus 35 mm.
True, 1:1 (sora has 34:34) lowest gear is absolutely enough for all the climbings on this route. I guess even 34:32 may be enough, depends of physical condition of rider.