Lineage OS is updated monthly. Also what are you talking about? The code is all available and under a libre license I might add. From a kernel perspective you can’t just port a newer kernel. That’s not how Android works. It has been done on some devices but it is very hard and usually very impossible for one reason or another.
From a signing perspective I don’t know but usually you get the firmware from the upstream manufacturer such as Qualcomm. From a security perspective phones aren’t very good as the modem runs its own software and is a total black box.