Things to do before Big Bang

High leveled 3-lined shields will be better than 20 attack shields depending on it's level. If it's a maple shield either the low level or the high level one would get curb stomped by either a 71+ or 121+ shield.

True, but not easily.

Going by the fact that a clean 3% str cape gave me almost exactly the same damage as a 4 attack PAC, I figure I would need a 15% STR unscrolled shield to replace a 20 attack one. Or a combination, say scrolled for 8 attack and cubed for 9% STR, or scrolled for 12 attack and cubed for 6% str. Possible, but not easy/cheap.
So, an already existing 20 attack shield is still quite valuable. But if one were going to create a new attack shield, a high level one might be a better starting point because it can, with plenty of money and luck, become much stronger.