So it should take about 8 million credits to quickly max out the new common/uncommon XX.

8Mil??? In what world lol I spent about 250K today on nothing but Basic packs and have almost every Common character at XX and a few Uncommon guys at XIII or higher and even managed to finish maxing the few Common weapons I had left and got one of the new variant weapons. (It was an M5-Phalanx Concussive which is trash but I got one lol)

I wouldn't. That's 6.6 million you could have used to put toward unlocking the nearly 700 new rares. That's, let's say, around 120 rare drops wasted and probably about 15 Ultra rare drops wasted. Just to get uncommon cards sooner?

By the time you unlock about 200 of the new rares via reserves packs, your uncommon character kits will be maxed.  Rares after less than a hundred.  As far as guns go... Not really worth it. When will you be using the uncommons over the rares at their max? Maybe the ones rtrnofdmax mentioned. That's it. Don't throw away drops.

I spent about ~300-400k to max my common characters. You get a lot of double cards in basic box (at least atm). I figured that it's a very good investment for infiltrators, sentinel and pwnytail. I will likely NOT try maxing out my uncommon manifest and move on directly to rares/URs. I don't think many uncommons are that amazing, especially in platinum, and for anything below that I can use either X rare cards or XX common. Although I bet AA could really wreck the room at XX in pre-plat, but oh well, maybe few years from now.

So what do you reckon is the best manifest building way right now?

Buying Basic Packs to max all common characters, then ignoring Advanced Packs and Expert Packs and go straight for the Reserve Packs? I've been mostly buying Experts until now to max my rares while getting more boosters, but getting those boosters seems nigh impossible now with the added silver weapon variants.

What are your thoughts on this, guys?

That.  The basic packs and drop pool are both small enough to be worth the investment.  About 20 million in cost now to max uncommons isn't worth it when you can put that too a lot of rares and ultras 

That's what I chose to do.  I've raised all of my common chars to XX.

Bit of background: I'd set a goal of getting everything through rares, never expecting to have a completely maxed manifest - and I recently achieved that.  Then I planned to do the individual weapon challenges, and continue to buy Premium Packs as I could afford them.

My current plan is to buy Advanced Packs until I have those chars at XX, and then go back to Premiums.  The RNG has always seemed to be weighted toward characters over weapons, but I'll probably end up with some variant uncommon weapons as well.  Shrug.