cost cap saved you from drowning, but it didn't teach you how to swim. You're right that the roller coaster wasn't a bidding problem - it was a creative problem. All cost cap did was stop you from bleeding cash while you were still throwing random stuff against the wall.
That bit about "20 creatives a day, 99% to trash" - that's the heart of it. Twenty a day isn't twenty tests. It's usually two or three ideas dressed up in twenty different outfits. No depth, no real learning. Just flooding the system and hoping something sticks. No wonder it collapsed every few days.
Now you've got predictability and profit, which is brilliant. But you've also hit the ceiling, because cost cap caps both the chaos and your upside at the same time.
When you're ready to scale again, don't fall back into the old habit of twenty disposable creatives. Instead, find the one or two messages that genuinely resonate, then rebuild those same messages in a hundred different wrappers. Face to camera, static, voiceover, UGC demo - same idea, fresh packaging. That's what separates scaling a winner from just flooding the account with fancy trash.
Back when you were doing twenty a day, were those all brand new ideas, or the same few concepts restyled?