How to set up feed-only Pmax campaigns reliably?
The approaches I’ve used so far (described here, here, and here) can be a hit or miss.
It’s no surprise since Google does not recommend this method for running a Pmax campaign.
The method I will show you always works because it’s an official feature of Google Ads.
Step 1: Prepare a standard shopping campaign that will eventually be converted into a feed-only Performance Max campaign.
Step 2: In the campaign settings, click Create Experiment to start a Performance Max versus Standard Shopping Campaign Experiment.

Configure the experiment. When prompted, select Create campaign for me.
‼️This step will replicate your standard shopping campaign as a feed-only (shopping-only) Performance Max campaign.

Step 3: Allow the experiment to run for a short period (a day or so).
Step 4: Find your experiment in the Campaigns, Experiments menu and click Apply.

This will remove the original standard shopping campaign and convert the Performance Max campaign into an independent, feed-only campaign.
That’s it!
This process has worked for me every time I used, but still, please let me know if you ran into any problems
Happy advertising,
Andrey