@JoshsOddCollection has already given some great responses!
I ran into some of these feelings when I just re-entered collecting some years back. The strongest I felt this, at the time, was wanting to start a Sekio artist collection and then completing it in a matter of days with a nominal amount of money and bulk ordering on TCGPlayer. Although I love their art, the act of completing the collection did feel a bit empty. I could have upped the difficulty by doing normal cards + reverse holos + any other variations, but I didn’t feel like diving that deep into it.
After that, I decided that I didn’t want to do massive buys on TCGPlayer to complete collections. I now try to approach artist collections a lot more slowly and naturally, picking up common cards when I happen to run into them in person or picking them out from pack openings (for modern artists). For those that basically only have vintage cards, I’ll pick up a card here and there on Ebay or some other platform—not being in a rush to finish or complete the collection. I find this to feel more rewarding in terms of the experience/journey.
As I write this, however, I do feel an urge to just update the Sekio collection to completion again using TCGPlayer, given that I do love the art and buying a few common singles anywhere but TCGPlayer seems inefficient
The vicissitudes of collecting, I guess!
Your species collection goal also somewhat reminds me of my own. Since I was just jumping back into collecting, I wanted to start small and only committed to English only and one of each art for my initial species collection. Once I finished that, I was inspired by other species collectors on the forum and moved to all variants of English + Japanese, with a few exceptions (no non-officially released cards and no cameos). This is where my species collection goal sits at today (for the TCG, at least), and I haven’t felt a strong urge to jump beyond it at the moment, given that I’ll need a good chunk of funds to stretch beyond this line (e.g. all languages or cameos).
As an complement, I’ve found non-TCG collecting to be very fun. There’s a ton of stuff out there (especially for Gen I and II), so you’ll likely never “finish” the collection, but there’s just so many awesome items out there to discover.
I think it’s great you’re already thinking about how you feel while you try and reach your collecting goals. Everyone’s journey is different, and everyone finds happiness/contentment in different ways, so it’s important to find out how to make collecting work for you 