LineageOS is not the answer. Check out the DivestOS, it is known for supporting 'out of support' devices. but I would strongly suggest you to change your device, that is too old. There is new vunrebility each month.
I am curious because I have been using LineageOS on Pixel 2 / Pixel 2 XL phones for a while (and it works very well, I might add), but I am kinda clueless when it comes to Android security.
Plus it just provides updates quicker, eg. the recent WebP CVE was first fixed in DivestOS before other aftermarket systems. https://divestos.org/pages/news