Privacy

Fairphone + Calyx should do the trick. Nowhere near as hardened as Pixel + GrapheneOS, but I think it's a close second option.

Is there a reason nobody mentions PostmarketOS?

As usual, the grapheneOS fanboys are replying. It's like asking for macs alternative and getting recommended macs with "but they are more secure and buying a used one doesn't give Apple money" comments.

LineageOS supports a bunch of phones. Some of them even support relocking the bootloader after flashing a new ROM (you'd have to look up which, I can't remember).

Then there's Murena which runs eOS.

Anti Commercial-AI license

I use /e/OS since 2-3 years on my Fairphone 3 and never looked back. Best is to just buy a phone directly from https://murena.com/ . You support the project and can be sure that the phone is supported for a longer period of time.

Graphene simply works better than other custom OS. Calyx and microG didn't work right compared to Graphene with Sandboxed Play which works quite well. Updates with Graphene are seemless vs major Android version updates on Lineage require more tinkering (I would assume Divest is similar). Graphene keeps your Pixel up to date vs some OS like e/OS are quite out dated. Also Graphene is very security hardened.

Graphene has a good reason they use Pixels and you'll have a better experience using Graphene on one.

If you're dead set on something else I'd suggest Divest. Look through they're list of phones

CalyxOS works great on Fairphones

Google Pixels with GrapheneOS were the only Android phones to withstand the backdoor software the FBI used to access the Trump shooter's phone.

Based on your stated assumptions about backdoors, how about building your own RISC-V phone?

https://mikrophone.net/

It’s not in a very usable state, but the hardware is open source.

Since you mentiin Fairphone s a example- get Fairphone. Not sure which ROM but it supports at least one of the options. I was thinking of getting one myself but it's unfortunately too pricey

I've been using the Fairphone 4 with CalyxOS and /e/OS before that, it's pretty neat

Pixel makes it easy. They aren't benefitting from you using it if you run a de-googled OS.

If you don't want to use a Pixel, pick your ROM, then choose from their list of supported phones.

And if you think we haven't considered hardware-based back doors, you may wanna revisit 1998 when we were all talking about this as a concern - that's when I first read about Android, before it was called Android.

Edit: Prior to the Pixel, I was running Lineage or DivestOS on an Essential Ph1 from 2018. I still have one that runs well on DivestOS. Not saying to get one, just an example of what's out there.

Also lookup the phones listed as compatible with a ROM on phonearena.com and gsmarena.com