If I'm understanding it correctly you want to carry 6kg of gear? In that case two panniers should be fine.
For a heavier load I personally prefer distributing the weight front and back.
Some would even say to put the two panniers on the front only.
If you're staying in hotels you won't be buying food for the evening.
You won't be meeting too many hills, certainly none steep enough to bother you. But, you may have issues carrying 4 panniers up to your room. Some old hotels have very narrow and steep stairs!
As for security outside shops etc. a bit of situational awareness goes a long way. I've always left my panniers on my bike (but I pack a lot more than 6kg). I normally take my handlebar bag though. I'd rarely lock my bike too. It's too darn heavy & awkward for someone to make a quick getaway!

But your situation is different. Two panniers 6kg total could quickly disappear.
I think, twice, (both times in France) I've pulled into a supermarket and left again because I didn't like the vibe.
For reference I've toured all over Belgium & NL and big parts of France.
As advised above, try out different setups - you've lots of time....and it's fun!
