What experiences have you had of winging it and booking on the day? We will be going south from Europoort and all the way to Croatia then back, probably taking 18 days in Europe.
I've always been doing this, both traveling by car and bike.
I would normally book in advance specific places I want to be in, of course, but generally everything else is booked on the day. Especially if wandering around.
I do travel every year with a friend (two bikes, two rooms) and we generally start looking at places to stop around 3/4pm, as you have an idea of how much more you want to ride.
Do not wait until you start being tired.
Easy.
Caveat: know your regional holidays.
I told the story already, but I had a couple of "incidents" in France over the years.
First one: was traveling south, couldn't find any hotels around the route (I generally always use Ibis in France for one night stops). Found one at the end, a few hours and a lot of miles over what I wanted to ride that day. Speaking at the reception I was told that I was traveling on the day where half of France transmigrate to the south by car. Noted.
Second time – this was silly of me as I should have known – we didn't book in advance a night for the 15th of August as we were moving from inland Provence towards the coast, me and girlfriend of the bike.
We wasted a day looking for a place, at some point coming back to our origin point. It wasn't an issue of cost, literally everything within 300km was booked. We ended up sleeping in the spare room of an hotel concierge home. She offered us that option as we were a bit desperate (past midnight) and I think she could do with an easy 80 euros.: D

Lesson learnt.
Apart from that, generally peachy. Especially using Booking. Sometimes you discover some interesting places as well. When traveling like this, I tend to stay in smaller and/or semi-isolated places.