I think it's much more likely that some business in a particular area will have a "good" road bike to rent if that area gets lots of visitors from far away (too far to bring their own bike on their own car) who really care about having a "good" road bike.
Most long-distance visitors to most
places in Europe (or anywhere) do not care about having a "good" bike. They're very happy with a mediocre bike.
Myself although I have a "good" bike at home, have enjoyed many happy hours of riding in Europe on mediocre rental bikes. Last year I rode up one of the toughest big climbs in the French Alps on a "city" bike with a kick-stand and only 7-gear hub-shifter and flat pedals with components made out of lead.
So . . .
Who are the riders who typically think they cannot possibly have a fun time on anything except a "good" bike: Racers.
Where do road-bike racers travel to? Famous stages of the TdF + maybe the Giro.
(e.g. Alpe d'Huez / Bourg d'Oisans + Mont Ventoux / Bedoin).
That's where to look for "good" road bikes.
What I have found is that local bike shops generally
are not good at advertising on the web. So they're even less good at advertising in a way that can be found and understood by cyclists who do not speak the local language.
So there's little point in asking about all of Europe, or three entire countries -- have to get real specific about which region and cities. Then maybe somebody can give you some clues about phrases to search for which signal "good road bike" in that language + region.
I remember once in the middle of the Dolomites finding a bike shop alongside the road, asked them about rentals, and they said, Sure we normally (but not guaranteed) have some expensive slightly used bikes available which we'd be glad to rent (and maybe then sell it to you if you like it). But there was nothing about it on their website -- you had to ask.
I see that a lot of people have the idea of going off to France renting bikes here and there and doing a stage of the Tour but it often doesn't seem to work out so well.
Where are you "seeing" that?
What problem did they have?
About three years ago my regular travel road bike broke down in a way that I couldn't repair until I would get home -- so I was able to rent a road bike (maybe not good enough for you
) in France -- and use it to do some famous climbs in northwest Italy.