.internal is ICANN and not (yet?) an IETF RFC:

* https://en.wikipedia.org/wiki/.internal

Other special use domains:

* https://en.wikipedia.org/wiki/Special-use_domain_name

* https://en.wikipedia.org/wiki/Top-level_domain#Reserved_doma...

* https://datatracker.ietf.org/doc/html/rfc6761

Looking at your links, subjective and legacy considerations aside, .alt per RFC 9476[1] seems as good a choice as .lan.

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