If you cannot split your opponents side on 3rd line and extend 2 spaces both ways then you should only shoulder-hit/4th line invasion?

Ignoring whether it’s the biggest move or whatever. If the side has no weaknesses to attack and you have to split if somehow but both sides are not 2-space-extendable. Is it safe to say I should only shoulder hit or invade high in general?

I’d say this is a good advice.

There is no such rule.

If your opponent has no weakness, even a two space extension or a high invasion could come under severe attack. In such cases you’ll want to opt for a lighter move. If your opponent spent several moves to fix her weaknesses, we can assume that in the meantime you scored points elsewhere and don’t need to erase her framework completely.

The following page points to several examples of reduction techniques (see links at the bottom):