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).

[1] https://www.rfc-editor.org/rfc/rfc9476