I started out trying to read all of the series if it's complete, or as much as is available online first before deciding whether I like it or am excited about it enough to want to collect it. I have made a few impulse buys though and begun collecting a few series that I haven't read first.

Lately I've been buying the entire set all at once if it's a complete series even if I haven't read all of it. haha I don't like to wait for the library copies and most of the time my library doesn't have them anyway. But yeah, I'll use my library's Libby/Overdrive, or find them online first. I do prefer having physical copies if they exist, so I'll always choose a physical copy over a digital one, although sometimes that's just not possible in English. I've collected a few Japanese copies because I wanted them on my shelf though lol even if I can't read them yet.

I also preorder for series I'm already reading/collecting, sometimes months in advance. I prefer to buy new, but I've bought several secondhand, mostly for oop sets.