Android

Pixel - varies by manufacturer

That was the Nexus line, Pixel phones are all made by Google. Although Pixel 5 series and older use Snapdragon SoCs, while 6 onwards use Google's custom Tensor based on Samsung's Exynos. The major downside is IMHO the awful modem efficiency - if I want to keep mobile network on so that I can receive calls, my 7a is limited to 2 days of battery life if I'm lucky (and that's with barely using the phone, just a few pictures).

Edit: and I forgot to mention that all Pixels have great third party ROM support, except if you want GrapheneOS, in which case you need to go for the recent ones that are still supported by Google.

If you want to use a custom rom, i can recommend pixels. Basically every custom rom is first and sometimes only available for pixel. I myself have a pixel 6a with lineageOS and magisk for root. Works fine apart from expected problems of tinkering with normally locked down files (be careful with ui customisation, got into a bootloop cause i wanted to change my quick tiles. Luckily easily fixed by deleting the settings folder of the app)

Edit: The app i mentioned was iconify which has a guide on what to do when bootlooping because of the app. (Need adb for that tho)

Pixel. GrapheneOS. No root. LOS and others with GApps have GApps with special privs.

Pixel is the best option, you dont need root these days for anything good, you can still root if you really want to but it defeats a lot of security that you'd otherwise have and for what gain?

I don't think any other phones would have a larger modding scene then Pixels either.

We have very similar device requirements lol. Though I can't speak much for the camera portion (I'm not a shutterbug and deal with whatever).

I just upgraded my trusty workhorse OP3 to a OP Nord N200. It's a few years old, but that usually helps since the custom ROM support is more mature.

OnePlus is usually pretty easy to unlock as long as you get an OEM model (e.g. not one branded / sold by a carrier). Been a while, but if you buy an OEM one, I think you can just unlock it without having to request an unlock code. I had to jump through hoops to convert this T-Mobile carrier model Nord into a global version, but after that I was able to unlock the bootloader with just the fastboot command (e.g. didn't have to request and flash the cust_unlock.bin. Was still carrier-locked to T-Mob, but that's who I use, so no biggie.

Similar to what you're seeing with ASUS, Motorola used to be, but I think their unlocking policy has gone downhill as of late. Haven't messed with their hardware for a while.

I haven't used Nothing Phone at all, but they were contenders (and still are) when I was looking for a successor to my OP3.

Same assessment of Fairphone: ideologically pure, but other issues ruled them out for me.

Look on lineageos.org for their list of supported phones maybe? I thought you wanted to program the camera itself, in which case you might have an easier time with a standalone camera than with a phone camera.