Funny - in some ways I have the opposite. In my version:

The iPhone SE would be the one I use for calls, SMS, etc. It has the SIM card.

The Pixel 9a would be used for everything I don't need a data plan/SIM card (browsing etc).

My needs are a bit different from yours. I like to separate telephony and communication (i.e. WhatsApp, SMS) from everything else. This way, if I want quiet, I just turn that phone to airplane mode. I really don't want to get random pings while I'm doing "real" stuff on my phone.