Android

I have used a lot of OnePlus devices, but can't recommend it since OnePlus has exited EU market. I doubt their firmware support continues as before. OxygenOS 16 seems to have issues on many models.

It's also getting harder and harder to root phones and get even basic Play Integrity to pass. Google and device manufacturers are closing up Android, and I don't like it at all. Some manufacturers (Samsung and Honor?) don't even allow unlocking the bootloader.

I don't know what I would go for next. I'd rather not but new phone at all, it seems old ones are better. But maybe second hand Pixel with GrapheneOS, at least it has manual native call recording.

We don't own the devices we buy anymore.

Personally I think OnePlus phones have gotten worse over the years, I switched from OnePlus 9 to a second-hand Pixel 8 and I didn't realize how much I would like the smaller size. I did install grapheneos too.

When I looked at specs between Pixel 8 and OnePlus 13 (both came out same year) I realized that pixel was slightly better at everything except a bit smaller battery size and slower charging which isn't an issue for me.

Regarding camera, I do use Google's camera app but I got Gcam service provider so I wouldn't need to use google photos as I prefer aves libre as gallery app, and photo quality was much better overall compared to my old op9.

In my opinion GrapheneOS, despite that not being its focus, is the most usable open source mobile OS, so that's what I use

As for the comparison between custom ROMs, you mainly have LineageOS and derivatives like e/OS, iodeOS, and CalyxOS, as well as GrapheneOS which is doing its own thing with Pixels (and future Motorola devices)

Base LineageOS is pretty much stock Android without Google services by default (though you can install microG). e/OS focuses more on the degoogling side of things, and they tie in Murena cloud services + their own iOS-like launcher too. iodeOS is another more privacy-focused spin of Lineage, and they have additional controls on content blocking (advertisements, trackers, that sort of thing), and you can optionally enable parental controls too. CalyxOS is a security-focused Lineage spin, though not to the extent of GrapheneOS. They only recently came back from the dead, but they seem like another good option too!

GrapheneOS is an uber secure version of Android, there's a table online for all the security features that Graphene has over the others, look at those and see if any of them are important to you. They also have a neat feature where apps requiring Google services can be sandboxed and kept separate from the system!

A custom ROM is not universal. GrapheneOS is focused on privacy and security. LineageOS on long support and others can also focus on customisability. So it all depends on your requirements.

The OP13 will probably be better long term in terms of performance. The Pixel phones are all 3 generations behind in performance compared to the contemporary Qualcomm chips.

You are in the UK, so that is a good start. If you want longevity, I would prefer the Fairphone 6, which has excellent battery, decent cameras, and amazing custom ROM support (+ mobile Linux is developing too!), and it will also be very repairable, so you can easily replace the battery, ports, etc. as they wear out. They also have a long 5 year warranty + 8 years of promised software updates, quite a bit longer than both OnePlus and Google. The main downside for the Fairphone is no telephoto camera, which is a bit of a bummer, but the rest of the specs are very apt for the price €599, or £519 in the UK, and I would say the extended support + custom ROM support makes it an excellent deal.

The Pixels are pretty terrible in all respects (thermal issues, poor battery, decent cameras but not competitive, etc.), with their only redeeming quality being that you can install Graphene on them (awesome if you prioritise security), and even that advantage is going away with Motorola soon releasing supported devices! I would avoid Pixels for the most part, unless you can score a deal on a used Pixel 8 or 9 series (barely different from the 10 really, the 8 is nicer if you want rounded edges, 9 if you want to iPhone 4 style boxy shape), or you really want GrapheneOS now for its better security.

OnePlus has great cameras and far more performance than anyone would need in a phone (hell, you can emulate some PC games on it with GameNative), and it supports LineageOS too. I've also heard that the 13 in particular has excellent battery life. It's a pretty nice option if you want performance and cameras!

You might also want to look at some of the Sony Xperia devices if you want a) headphone jack! b) no notch/cutout! c) front-firing speakers d) great cameras and e) microSD card slot! The Xperia line is also relatively compact, if you want a smaller phone.