With @GrapheneOS and @calyxos only really supporting Pixels, there isn't a good ungoogled option that doesn't use Google phones.
Don't get me wrong, Google hardware is fine to use, but it cuts down on Android's customisation. If you want an sd-slot, extra-sensors, etc. you either harden @LineageOS yourself or look for GSIs, which are currently dying out. Between bootloader unlocks becoming more complex again and phh stepping back.

GrapheneOS is a very different thing from CalyxOS and LineageOS. Both of those roll back security rather than improving it. They aren't hardened operating systems and therefore don't have comparable security or privacy features either. It's a misconception that they're providing at all the same thing.

Hardware and firmware security are important as are hardware-based security features we use to defend the device. Those requirements are listed at https://grapheneos.org/faq#future-devices.

Most Android hardware has very poor security. The non-Pixel hardware that's closest to meeting our requirements either bans using an alternate OS or cripples the device if you use one as Samsung does, including crippling security features.

https://grapheneos.org/features is a list of the features we add compared to standard Android 15 QPR1.

See https://eylenburg.github.io/android_comparison.htm and https://eylenburg.github.io/old/android_comparison_2024old.htm for a 3rd party comparison table which helps understand the difference.

Quickly moving to new stable releases is essential. Contrary to what many believe, older Android releases only get partial privacy/security backports. Only latest stable releases have real long term support. They go through long public betas before stable releases so they're not bleeding edge. Android 16 is being publicly tested already, as is the next quarterly releases of Android 15 (Android 15 QPR2). Quarterly releases are nearly as large as yearly ones under the hood now.

We have a thread about DivestOS and a potential replacement for it which does not yet exist at https://grapheneos.social/@GrapheneOS/113772409415058393.

A proper, modern replacement for DivestOS would simply be a port of GrapheneOS to other hardware including end-of-life devices with partial security patches and a subset of the security features available. Not something fitting into our goals, but others can do it.

Can see from the older comparison table we linked above that others aren't the same at all.

I completely get your point, I'm just lamenting the fact that this brings Android closer to being a monopoly again (forming a de-facto smartphone duopoly.) The hardware features supported by GrapheneOS thus become a (sub)set of what Google decides to implement in their phones.
Play Integrity is already an overreach, similar to the attempt to spread the browser-attestation concept.

You seem to wrongly believe GrapheneOS is written specifically for Pixels. No, other companies are not trying to build reasonably secure hardware and aren't providing proper security patches or the standard security features. There's no actual barriers to other companies trying to provide good security, they simply aren't doing it because it doesn't sell phones and they don't care about it. GrapheneOS will enthusiastically support another secure device as soon as one exists.