I mean, you might cover some vulnerabilities that were discovered after the manufacturer stopped updating your device, which is nice. But only time will tell what new vulnerabilies will be uncovered next; but be sure, they will.

Only a frequently updated device will have constant state-of-the-art vulnerability protection. That is, until the maintaner (someone with the know-how to make stable lineage-os builds and mess with the device's vendor tree doing all this work for free) decides to stop updating that device. Which sounds bad but that doesn't stop another maintainer from rising up to the task eventually.

Anyhow, with lineage and, generally, any custom OS aimed at phones that can't relock their bootloader safely you'll always lose device integrity (can be circumvented with things like magisk) and very likely IMS features (VoLTE and the like).

Another thing to consider is if your device ends up in the hands of a malicious party. If its bootloader is unlocked, you can be sure they'll have easy access to any personal data inside it.

If you wanna be safe for a looong time I'd consider a pixel phone from this list and flashing grapheneos and then relocking the bootloader.

In any case, good luck and all the best to you! :)

Sidenote: if you are on a Linux system and do intend to flash a custom recovery (necessary step before flashing a custom OS) on a samsung phone, take a look at the Heimdall tool. It's an open source alternative to Odin that runs natively on Linux.