To answer your question: of course a road bike is great for a 200km or longer brevet! And, I think your bicycle is great too. In my opinion, to get better long rides on your arms and rear the order of most to least expensive and effective changes are new fork, tires, then handlebars and stem.
The fork on the 520 C1 is steep, short, and has almost no rake (or fork offset). Here's an illustration:
Your frame has a pretty slack head tube for a road bike (good for you!), so you probably don't need to go all out; but a longer fork makes a shallower headtube angle which is going to stabilize your ride and require less work which is going to save you that arm and shoulder discomfort. On top of being steep and short, the fork doesn't have any rake (or fork offset) to absorb road discomfort. If you get a fork with more rake, it'll soak up some of the road vibrations, potholes, pebbles, etc. and it won't all be going to your rear end through the seat.
This article says your frame can take up to 32mm tires. The wider the tire, the less pain in your arms and ass. B'Twin's website says stock width is 25mm, very skinny. If you don't want to go to 32mm to save your butt, 28mm is a common size.
There's been some great advice about handlebar height! Swapping the handlebars would be a much larger project than swapping the stem. A little more rise is a good way of working on that back pain and longer drops than your compact bars offer might give your shoulders and back some relief.
tl;dr get a fork that isn't built for crits and twitchy, short rides.