I have 10y experience running omnichannel for clients.
First, you figure out how much data you are losing due to cookies (most tools can track % of approvals), and you of course modify the cookie acceptance in a way to increase chances - middle of the screen, have "preferences" instead of "decline", etc. Ideally you get around 90% acceptance.
Then, you establish your trends (GA4 is as good for this as any, as it is not about the exact attributed numbers). Say you got 1 mil in sales, and GA4 says:
40% Google paid
10% Google organic
20% Meta paid
15% Meta organic
5% Bing
10% some other bullshit
What you do is you monitor these ratios, say you increase your meta spend by 25% - you see sales increase in your ecom system, and here in this attribution model you will see that meta stayed the same or with just a slight increase, but google went up, meaning people are searching what they've seen in your ads.
The key takeaway: focus on trends/ratios, not exact figures.