Android is pretty secure. The kernel is highly stripped down, so it is more like a microkernel. Like, removing support for most filesystems etc.
They also moved a ton of things into userspace, I think their entire filesystem uses fuse.
Android is really good.
But still, LineageOS may implement all the minimum AOSP patches. But Google Pixel phones get more than those.
And monthly is of course slower than weekly, bjt expected if you support such a vast range of devices.
Then the issue of course is, is the firmware still supported? Where do the kernels come from?
These are likely out of reach for the devs.
Also, their OS is very generic, just AOSP with mostly cosmetic changes.
Where GrapheneOS implements tons of deep modifications, which makes updates and maintenance a bigger task. These features also cause more bugs which need fixing.