The two PRIMECAP funds that I see are both closed to new investors, so except for your own education, the question is irrelevant. Beyond that, you can look to see what index that fund follows, and attempt to find another that tracks that same index if you're really set on it.
The idea behind using the total stock market fund is that you're probably not smart enough to pick the fund that's going to beat the market. That's not to say you can't experiment and enjoy the journey, but the one thing you can absolutely control is expense ratio, so total stock market is very attractive from that standpoint. Also, you get to (more or less) MATCH the market, since you probably can't beat it.
Also, FIRE means Financially Independent, REtired. I'm learning a lot here, too. Been reading a while, but this is my first post.
Forgive any typos, I posted from my phone.
Adrian