Dealing with PPC traffic cannibalization

I've been in e-commerce for a while now and have always had very mixed experiences with paid traffic.

I decided to take another run at Amazon ads for about a month starting in mid-January. I sell t-shirts & posters and am getting about $35 AOV at about 50% gross margin.

After 26 days, Amazon was showing around $100 a day in PPC sales at 25% ACoS. Not great, but still profitable... until I calculate the actual sales lift. Comparing to time periods when ads weren't running, it looks like I've actually only gained about $40/day in sales. The other $60/day I would have gotten anyway and am now just paying an extra 25% in fees for it. Once this is factored in, I'm actually losing money on the ads. That wouldn't be so bad if it was building up my own customer base, but since this is Amazon I don't really get that benefit. So I shut the ads off.

I have similar experiences with Google ads. I can never seem to get the minimum needed ROAS after accounting for cannibalization. How do you deal with traffic cannibalization? Is there a better way to measure it, and is there any way to mitigate it?