One thing you could do is leave your game sitting for a couple hours with 2 blue plates and 1 pink, and switch the pink to a blue so that you get 3 4x, freeing up another slot. This will take a few hours, maybe just 3, but that's the strat I'm going with
Yes, currently I am up to 8 plate forge slots, 4 upgrades in forge speed, and 2 upgrades in yield so I am able to get to one 4x multiplier after waiting about 15-20 minutes, which gives me enough blue plates to buy the game boosters after another 10 minutes or so. That way I end up with 8 game boosters. I was trying to include in this in an edit to my original post, but itch wasn't cooperating lol. Basically, as you get more plate forges and more upgrades try to reduce the number of multipliers you buy to get the maximum number of game boosters possible