I also relate to this. Born in 1981. Most of my life I just assumed I was gen X, but then reading the actual definition a few people put the last year of gen X at 1981, but most put that as the first Millennial year.

As I have aged I have embraced more of my millennial characteristics, but still feel like I fall more on the gen X side of things.

Xennials in particular have a very interesting connection to very specific cartoons and video games from their youth, I have found, and they are somewhat unique to Xennials.

I think you are onto something with the 5-year changes in categorization.

Another issue I have with the whole generational theory is that Baby Boomers generation is based on actual birth trends and lasts longer that the generations after it. X, Millennial and Z just get fifteen years that arbitrarily end with no reasoning. Boomers start and end with the actual "baby boom".