My house has a Square D Load center likely installed in the 70s. It's only a 12 space box, which is not enough for my needs. I have 100 amp service, and a residential load calculation based on the future setup of the house came in around 68 Amps, IIRC. So there's headroom. (Small 900 SF House, no Basement, gas range, gas dryer, gas water heater.)

Anyway, I want to add a subpanel so I can have the appropriate number of circuits.

So I have 2 questions as I try and figure out the feeder size for the subpanel.

How do I interpret this sticker? Does it mean ONLY Use 8-14 gauge wire, and ONLY solid? Or does it mean I can use larger than 8 gauge, but if it's 8 or smaller it must be solid wire?

If I am limited to 8 gauge, the ampacity of the feeder is limited to 40 amps unless then connections and wire are all rated for 75C instead of 60, correct?
If so, can I interpret this sticker to mean that the connections made in the box w/ CU wire are definitely rated for 75C, so I can run a 50 amp breaker instead of 40?

And, if I am indeed limited to 8 gauge on the neutral, does that limit me to 40 or 50 amps, or can I run 6 gauge Hots and an 8 gauge Neutral to the subpanel and get a 55 or 65 amp rating?

Just in the planning stage now, but trying to finalize plans. Can't do that until I know what load the subpanel can carry. Then I can figure out which circuit will go in which box.

I hope I've provided enough info. Thanks!