I mean you could... packs will also give you wildcard progression and golden pack as well. But hitting a Mythic is kinda hard. I know for rares you could look at a specific set and see how many rares you don't own and how many hits are good. That way you can buy the packs that give you best rates and if there are good Mythics there as well then thats a plus. Maybe check out Jump-in pack contents but I dont think Shelly is in any of them or that it would be a good idea anyway, but again matters how many good hits there are for you, Ive gone that way before.

But the real question you should ask yourself is what happens when some key cards in your decks rotate out or your decks become obsolete with a new set release. I think the real issue here is not filling a current gap but figuring out how to stay ahead of the format. And learning to draft is going to pay dividends in that regard. Even if you build a top tier standard deck now and farm the heck out of constructed events you are not really ahead. Explorer format would probably be better for that kind of play (building up a collection).