Some of us are already doing this. My main phone is a Google Pixel 8 running LineageOS 23.2 with F-Droid, microG, and Aurora Store installed.

For things requiring Play Integrity, I picked up a $20 burner carrier-locked Motorola phone at Walmart for $30. It's WiFi-only, given that I'm never going to pay for service on it, but I can also tether it to my main phone. It's also useful for writing one-star reviews on apps that require Play Integrity to function, which is something everyone should be doing.

this is basically what I do now.

gmail and a "work-ish" phone. official stuff like DMV or banks use this. my work requires MFA and auth apps and they live here too. no SIM card and mostly lives at my desk.

my main phone for doin stuff is a different phone with a custom ROM and nothin but f-droid.

>and the "real phone" from aliexpress or whatever that is purchased rooted and you actually live your real life upon.

Ironically the phones with best third party rom support are google pixels. Good luck getting lineageos support or even unlocked bootloader on a random aliexpress phone. You might be able to sideload without restriction, but the ROM is probably gimped, won't receive updates, and has random privileged apps possibly spying on you.