Ask HN: Is NextCloud too heavy for R-pi 3 with Docker?

I actually know the answer to this, but instead of just telling you, I'm going to instead suggest that you try it out and see for yourself and see how it goes. Since you have the Pi already, there is literally no downside to giving it a go, aside from a few hours of your time, which will not have been wasted whether it works out or not.

You are going to learn a lot more that way than reading dozens of replies containing contradictory opinions and inane pedantic arguments over tangents that these kind of questions tend to attract.

Good luck.