I've been a (happy) Lineage OS user since 2013, and Pixel 2 is still supported by them, so you can check them out. But note that the security updates provided by Lineage OS only pertain to the Android part, not the modem part. This isn't something that a third party OS vendor can fix, as the modem/firmware updates are supplied by the device vendor. And even those are reliant on their suppliers, so ultimately unless they have the negotion power of a multi trillion dollar company (not every vendor has that), they don't have the power to support devices for a long time either (see Fairphone's struggles with qualcomm for example).

For Lineage OS, they maintain the kernels released by the vendors and backport patches to them. But this often means that the backports only address the publicized CVEs and might gloss over a lot of changes between kernel versions that might not have had famous bugfixes. A lot of patches don't end up in a kernel, and often while the version number reads of that of a modern kernel, only a small percentage of the patches in that kernel release was applied compared to the upstream kernel. For this it's important to note that the kernel doesn't have a clean separation between "bugfix" and "CVE fix" for patches, this is already a problem that their officially maintained LTS versions have.

From the security point of view you are definitely better off than staying on something that's unpatched, but I have to mirror what parker_mountain is saying. It's way _way_ better to just get an iPhone as that also gives you modem security updates for half a decade. I am really unhappy about the lack of freedom these devices give me, but I have switched to an SE one year ago. It's just such an incredibly good TCO for really good security.