I’d start by splitting the Q4 budget across 2–3 traffic sources instead of relying on one expensive channel. Test 3–5 creatives early, cut anything that isn’t converting, and move budget toward the campaigns giving you the lowest CPA or strongest ROAS.
Also, don’t wait until December to optimize. Set weekly targets for CPC, conversion rate, CPA, and ROAS, then adjust based on those numbers. Keeping 15–20% of the budget flexible can help you scale the campaigns that actually start working during the holiday rush.