1) It’s a bit too late for the 5:
https://endoflife.date/pixel
https://grapheneos.org/faq#supported-devices
https://grapheneos.org/faq#recommended-devices
2) In which country?
You can buy a SIM anonymously, that’s the easy part, but you won’t be anonymous while using it, that is if you use it where you’re known to be (e.g. at your home or workplace). If regulations in your country demand an ID for any SIM card purchase, you can for example ask someone to buy it for you. Prepaid plans would be better if you can add money without an ID tied to the transaction.
3) Do not expect to juggle with several ‘strong’ identities with only one device. Beware of hardware identifiers and non-hardware identifiers, with the Media DRM ID being somewhere between those two categories.
4) This will depend on your threat model…
Maybe one location you should avoid would be your home … cannot tell.
Maybe any places with cameras – filming your activities on your phones, your code and so on.
Maybe usually crowded places, which can have additional spying equipment, besides cellular antennas themselves.
Maybe airports where custom could try something with your device.
Maybe no place in particular.
Too many considerations.