I can't speak with absolute authority since my experience is limited to 35 runs but, I think I have discovered a few things.
First, outside the legendary character pieces which drop most commonly at 25K (about 85% in my experience) everything drops equally at 15K. Basically, for people hunting the Super Rare and Rare accessories it is a waste of time to get 25k after obtaining all of a particular side's party characters. 7.5k does drop items but seems to favor Rare items, so collecting Super Rare items is best done at 15k.
Speaking of, this may be common knowledge but plant items only drop as the dino and zombie items as the cat.
Also, there seems to be special items that do not appear on the sticker book until obtained. I made a checklist for the plant side only to receive a special hat for the rose which is displayed between IT48 and IT49. Which annoys me because there is now no definitive way to confirm I collected everything from Infinity Time. (Since special items I don't have can just pop in wherever they feel like.)
It also seems to me that outside hunting legendary party stickers, it is a waste of coins to buy Infinity Packs. (I bet EA will be upset I reveal this.) Doing a run to 15K should take about 20 minutes, and rewards ~22K coins if you defeat the Red Boss. (No reason not to, even though it is possible to reach 15k during wave 20, without defeating him.) At 200K per pack it is roughly 65K per IT piece, if you factor in the money you make while farming, you are losing ~85K for 20 minutes of work. Now, if someone knows a way to earn 85k in 20 minutes I would love to know. Otherwise, it is a "waste" of money to buy Infinity Packs. (You spend more time earning the money to buy the packs than you would just farming the items.) Of course, for people who can't stand Infinity Time it may be worth the coins, but for everyone else I wouldn't recommend it.
*Note* - I am aware that players are not guaranteed an Infinity Item each run, increasing the amount of time each item takes. However, in 35 runs of over 15k shards I have gotten coins twice. As of now the percentage seems too low to make a major difference. This also doesn't take into consideration the loss of experience, it is strictly a measurement of efficient spending.
Overall, for anyone just looking for Party Variants it is not a very large investment. In fact, I would bet it is more work to earn the coins to find Party characters in Infinity Packs than to just unlock them. Getting 25K has a very high chance of character drop, from what I understand packs are far less generous. I encourage anyone having trouble to look up a best practices video, they are very helpful. Hope that helps.