Looking at your links, subjective and legacy considerations aside, .alt per RFC 9476[1] seems as good a choice as .lan.
Personally, I just use a registered domain for this purpose, as they're cheap enough to not care (last I checked, I pay $10–20/domain/year for registrations, depending on registrar and TLD, and $0.21/zone/month for hosting public zones on Google Cloud).
A registered domain has the added benefit that you can use the same namespace for externally accessible services. I personally do this (using Cloudflare Tunnels, not open ports).
