You need GrapheneOS to sever the link to Google. You can also deny specify apps and services Internet access.

Is android auto still available with Graphene? AA is genuinely one of the few life-changing features introduced in the last decade that I'd prefer not to go without.

Mostly works, some stuff doesn't. The worst thing that doesn't work is alternative maps (e.g. OsmAnd).

Yep and works flawlessly via USB for me. That was a deal breaker for me for the longest time too.

Allowing it to connect over Bluetooth requires granting AA plenty of additional permissions which I didn't want to do (but hey, on GOS at least you can muzzle that thing).

I like the idea of graphene, but I worry my banking / brokerage apps wouldn't work anymore and that'd be a deal breaker

The Graphene community maintains a list of compatible banking apps.

Another possibility is to keep an old/cheap, stock Android phone at home with WiFi only for apps like this.

Doesn’t that defeat the point of using an app at all? Use a computer at that point.