As far as how many sets you should collect at a time, it’s really up to you. What I do is if I only have 6 free packs is to just focus on two or three sets at a time—by doing it this way, I’m able to get as many cards as I possibly can from a series at a time, and therefore increase my potential for trading. I mean, you’re welcome to try and collect as many different sets as possible at one time, but the downside will be that you won’t have too many cards available to trade from any one collection and thus will be unable to quickly grow your collection, especially with larger series. Basically less stuff to trade.
If you get a streak of opening packs and can get the extra +3 pack reward, you’re able to collect the most from more series, especially if you come back to the site later in the day. For example, I was opening the maximum number of packs from 6 different series daily at one time. Like having my cake and eating it too. -edit- Doing it this way also will make it potentially easier to trade across different series. From experience.
About credits, that’s also up to you. Some older sets like LET’S PARTY have run out of free packs and credit packs are the only way of getting those cards, barring expert trading. Sometimes using credits will give you perks when using them to open packs, like at least getting cards of a certain rarity or boosting your odds. These days, you can pretty much complete sets for free. (Well, you always could, but now it’s WAY easier) It will just take longer. I’ve used credits to complete old half-finished sets and to get prints from old sets I really liked.
Also that strategy thread silver linked is pretty handy. I found matthberg’s comment about calculating the print value of multiple prints in a trade to be suuper helpful. I’d put it in my trader bio but it’s too long.