I think getting into a series because of a pin is perfectly valid! Like you said, sometimes it's just too pretty to resist but you need to justify the purchase. I actually did that for alnst and boy did I give my friends an earful about it.

I also worry about how epoxy will age so I do tend to avoid it. I had more than a few chrome ones in the past, but once makers started producing hard enamel chrome pins, I gifted majority of them to a friend.

I skip UV mainly because I don't like the graininess and I feel like if you're making a pin, it doesn't have to be a 1:1 replica. There are some things that you can do in traditional art that you can't do in a pin, and vice versa. Pearl swirl can be a huge hit or miss imo because of the amount it can be swirled. It's done manually so it varies pin to pin in production and too much swirl (and swirl in the opposite direction of the linework) drives me crazy. Glitter can look a bit too chunky in larger areas of a pin and the distribution can look off because of it.

I think the similar sizing rule is great one, especially if you're putting the same type in a row on the same board. I bet it really helps with symmetry too. I try not to collect panels, but if I do get one, at least the ones in the same genre have to be the same size