Have you considered Parabola? Its libre arch, and one of the best reasons to own those old thinkpads. Its glorious.
Discussion
Is the only difference the kernel, i.e. linux-libre?
If so, can't I just install that kernel from regular arch?
Its kernel, libre whitelisted repos, and a few other small tweaks.
I realize I just assumed you're running libreboot firmware on the laptop, but that may not be true. Worth checking out, if not.
Last time I checked, i don't think you can libreboot a t480, maybe I'm wrong.
I'm all for using libre software and prefer it when it exists but at the same time I don't like being artificially restricted. Cool idea though!
Oh I thought the T480 was a Core2Core Duo variant... I was mistaken. My bad 😅