I don’t know what the state of either is on a Sony 10 IV, but the answer is almost certainly Lineage. It’s the more mature project, and it’s way more likely to work. And at the risk of getting on a high horse, Lineage’s unfortunate but necessary (if you actually want your phone to work) reliance on proprietary blobs is already bad enough; I certainly wouldn’t trust my privacy to a project like Sailfish, which I understand to be partially closed source.

I mean, you don’t necessarily need a fresh Arch install; I’d wager you could just switch to Arch in place by editing your /etc/pacman.conf and /etc/pacman.d/mirrorlist to Arch repos. You’d probably need to keep a Manjaro section at the bottom of pacman.conf to avoid breaking too many things at first. Is this a good idea? Probably not, but it’d be fun to try.