if I invest in Bitcoin today, am I still an early adopter?
I think invest is the wrong term. Investments have some kind of builtin return like interest or dividends. You're buying bitcoin in the same way you buy gold or a lottery ticket. Maybe you have some solid reasoning on why price will go up but you aren't getting any returns like in an investment.
Suppose I buy gold to diversify my portfolio, because it has a low correlation with the other asset classes I own? Is that an investment?
Crypto can play a similar role.