I am sorry. I was not clear about the steps.

Close Outlook.

Follow these instruction to Add a new Outlook profile named something like "TEST 2025-03-19". Don't touch your existing profile(s).

How do I create a new Outlook profile

Select "Prompt for a profile to use".

OK your way out of the Mail Control Panel dialog.

Start Outlook. It will prompt you for a Profile. Select the new TEST profile.

Go thru the initial Outlook setup. Once that is done and you are at the Inbox, choose File -> Open -> Open Outlook Data File. Select the single PST file that you want to use.

At this point you should have a TEST profile with only the one PST that you want.

Check that everything is working as you expect. If everything is OK, then Close Outlook.

Open the Control Panel Mail app again. Rename the TEST Profile to something meaningful. Select this profile and choose "Always use this profile".