When looking for star notes, I always keep an eye out for X serial number enders and today I got a beat up one. Looking around, I see that some thread here back in 2017 mentioned Y is even more rare than X (assuming they print from A to Y excluding Z and O), Y should be more rare than X right?
Yes. The BEP will run through X to get to Y, and Y is typically only a partial run with less notes printed.
IMO the series 2009 L -X note was the most collectible with only 12,800,000 printed. I've got 6 of them but all in circulated condition.