I beat the entire Grasslands, which took me approx 8 hours. None of it felt like filler actually. There is just about enough variation and the majority of quests are short enough. There is one that goes on for quite a bit, but that one is rather fun.
The Grasslands was 10/10 for me in terms of atmosphere, and Kalm is a excellent town to start in. Sidequests quality is improved, still nothing memorable but better than it was. That inkeeper guy has some backstory to him if you continue with side content.
The problem might be that the second area seems to repeat the same tropes, find a chocobo again, do towers again, find the regional summon data... etc. So the open world might grow weary eventually. But there is always the main story you can follow, and so far it delivers. Another issue is that, probably since I did all side content, the game is too easy. It really is far easier than Remake was, there is more room for error and more ways to end fights quickly. I might switch to Dynamic.