FlandreOnDuty
You should look for an 8 or 9 Pixel, because it's not fully confirmed yet that 10 will be supported.
If money is a question, you should not blindly buy a Pixel just to have the opportunity to install Graphene.
Better save your money and when the time comes that your old phone needs to be replaced, you could look out for a new phone that supports it. As i mentioned, there is good chances that besides Pixel there will be another Smartphone model that will support GrapheneOS.
I would not suggest buying a new phone when your old one is still okay to use, GrapheneOS is working great, but in the end it's still just a smartphone experience like any other (which is good, because it is super stable).
Basically you will have a phone that has no AI and no wireless payment, and some banking apps will not work either. And none of the Google features that are integrated in the stock OS Pixels get shipped with. Besides that you will have probably the same experience like you will have it on your de bloated Xiaomi.
The battery lifetime of Pixels is always in the midfield. On standby you get about 3-4 days, with average usage you get 1 to 2 days, so it will not be better as your current phone. You better overthink the decision to buy a phone directly, if i were you, i would buy a new phone only if necessary.