It sounds as though you're intending to tour around, rather than stay in one place. So the first question to ask is what sort of accommodation you plan to use - hotels, B&B's, camping, friends? Depending on the answer, you need to do a bit of research and maybe book ahead, though in June it shouldn't be too busy. But be aware that in many places campsites are often only open in high season. And if you're camping, maybe have a plan B for accommodation in case the weather really turns against you - in June it can be almost anything in the places you mention, particularly in the hillier bits.
Secondly - are you going mainly to enjoy the riding or for sightseeing? IMHO it can be difficult to combine both satisfactorily - for sightseeing you tend to want to be moving around all the time, whereas to enjoy riding it's generally better to base yourself in one place for a few days, leave the luggage behind in the digs and go explore. So if you do want to do both, maybe plan to split the trip into two halves. And if you want to enjoy the riding, I'd suggest the general area to aim for would be the Ardennes/Eiffel areas - German roads are generally better surfaced than Belgian ones, and in particular the ones around the Nurburgring are brilliant - you don't have to go on the 'Ring to have fun!
Security is something to consider, particularly if you'll be leaving the bike in urban areas while you look around. Both the bike and luggage can be vulnerable.
Preparation shouln't be a big deal with a new-ish bike - just make sure it's all lubed and adjusted properly, with plenty of tyre tread left. Get recovery, but only take the essential tools to make any adjustments you think might be necessary - local dealers may not be able to fit you in quickly at that time of year when everyone's getting their bikes ready for summer. A puncture plugger is a good insurance policy, and I like to take a small first-aid kit. But in general, travel as light as possible. If you're going with others you can share out all the bits and pieces.
Make sure you have all the necessary documentation and stuff that needs to be carried in the countries you'll be visiting, For example, as of this year, in France bikes have to carry a hi-vis bib to wear in the event of a breakdown. Also helmet reflectors have been compulsory there for some time, though I've never heard of anyone having trouble by not having them, and in theory it's still necessary to carry a breathaliser, but they aren't penalising people without - logical?
Have a great trip!
