Is there any particular scenes (do provide time track) where the pixelisation is more obvious? To my eyes, your video looks good. But then again, I'm using a small 12" laptop. Perhaps others with bigger screen can chime in. As the other member has mentioned, it could be YouTube playback compression. So don't let this worrying you too much.
I haven't tried
Vimeo yet, not sure it will be better.
By the way, great work on your video. The scenes are absolutely gorgeous! Hopefully, prior to the expiry of my lifetime, I'd able to pay a visit to the capital of Greece.
Was the camera setting in auto? Try using CP/ND filters for bright scenes. It will greatly enhance your video footage. Hope to see your next video soon.