I've just been randomly picking cards to do my daily quests (no strategy at all, total newb, lol)

Pretty much the same for me when I first started playing the game. It took me like 10 matches to finally figure out why my ranged monsters are not attacking (I placed them at the very front at that time). XD

what's the optimal strategy for opening bought booster packs? Is there any difference in opening them now vs waiting to open them later? Is there any disadvantage to opening them immediately upon purchase that I don't know about?

I suggest buying specific cards (that you really need and will often use) on the market rather than buying packs. I maybe somewhat biased here though since I haven't had that much luck on opening booster packs but I personally think it would be better to level up cards that you will often use than to rely on the randomness of the booster packs.

There's a promo this week though so if you are planning to buy a lot of booster packs, this week is probably the best week to do that.

As far as I know for booster packs, there isn't really that much difference between opening them now or later (some however convert this packs to tokens and wait for months before selling them again in the market at a much higher price).

But if you want to get a 'somewhat' higher chance of getting a gold-foil card or a legendary then it's probably better to open them with some potions in hand (rarely works on me though T_T). I don't suggest buying the potions but the reward potions you will get from the daily quests is enough.