Yeh it’s what you are good at and interested in surely.
DS2 is pretty able, but would have found A level Eng lit much more of a challenge than the biology he took.
Seems for best chance of RG degree you need to be aiming for 3A predictions I mean this is true to some extent I guess, but it massively depends on the degree and which RG. If you mean Oxford for anything or LSE for economics yes, add stars too maybe.
If you mean Liverpool or Newcastle for Eng lit or history then ABB will probably do. That was DD's offer for Eng lit at Birmingham, actually.
I think the best advice is to consider future careers you might be interested in (for example, if you want to be an engineer, you will need physics and maths A levels ideally) and make sure your A level choices don't rule them out.
If the future career question is answered with "hmm not sure" then the best thing is to choose subjects you enjoy as that is how you will get the best grades – which is important. If a student really doesn't know, then choosing one or two core subjects is a good idea as it leaves more options open. So for an undecided student, I would advocate for (for example) a trio including history and Eng lit or chemistry and maths. Facilitating subjects are no longer something that unis look for, but those core subjects are more likely to be needed for a degree than (just as a random example) law A level, which no uni specifies for any course.