In order to unlock the Royal content/Third semester, do I only need to...

[A]

Get Maruki's confidant rank to 9 before November?

I don't want to read the guides, but I'm worried about missing the content by choosing wrong or ignoring something.

[Nibbie]

Yes. There are other confidants you may need to rank for extra scenes or features in that third semester, but unlocking it just requires Councillor being levelled. Granted, that confidant gives great bonuses, so levelling it makes sense anyway.

[Radiosity]

Kasumi to rank 5 (which is her max until third semester), Detective-kun to rank 8, Maruki to rank 9. That'll give you third semester and the true ending stuff. Make sure to have them ranked up before December.

You might also want to focus on the Sun confidant and get that maxed before November.