Can polish be a game changer, something that makes a boring game fun?
It definitely can. Several years ago, I was part of a community that did yearly game development contests. Entrants had 3 months to develop whatever they wanted, and each entry was judged in several categories, like gameplay, art, etc. One year, someone entered a game that had unusually shallow gameplay - he self-described it as a "hold-the-button clone", where the game basically consisted of holding down the mouse button on an object for a while with almost no other interaction. However, he put a huge amount of effort into the presentation, with impeccably polished art, sound effects, animations, and everything else. Everyone loved it, and it did well in almost every category that was judged. This was before the days where "clicker games" existed, but that's probably another good example of this phenomenon.
At this point, it feels like I'm just pulling random ideas out of my butt, implementing them, seeing how people react, and repeating. But the goalpost of "fun game," never seems to get any closer. This has made me hesitant to make any huge additions, since nothing small I work on seems to help, I feel like working on something big could be a huge time sink with no payoff.
Is the game fun for
you to play? I feel like that's the most important first step. When you hit on a core loop that deeply resonates with you, that gives you intrinsic motivation to polish it up in a way that makes it more fun for the kernel of an idea that you can see inside it. Bridging the gap between "fun for me" and "fun for everyone else who likes the types of things I like" sounds easier to me than to try to figure out from feedback how to make a not fun experience into a fun one.
Sometimes an idea just doesn't work out. It's unpleasant, but the sooner you can discover this and move on, the less time you waste on something that's ultimately not going anywhere. Personally, I've found that I can often fall into a trap on a project where I get too entrenched in certain decisions I've made about the game early on. For my current project, I've been very deliberately keeping it flexible, and giving myself permission to reinvent large swaths of game mechanics if I think I can do them better a second time. As long as I have that internal driving force of "this is cool, but I'd like it more if it were like THIS instead", I know I'm still on the right track. If I find myself committed to something that just makes me feel apathetic, it's a sign that I'm drifting off course and need to rethink some core part of my design.
I don't have any more specific advice, but maybe this will help somehow?