You've only had about 16 years since the formalization of RFC 2606 [1] as a best current practice to move your internal DNS over to a namespace you actually control.

Yes, it's most likely going to be more typing, but this way, you're not risking having everything break at some point when your pseudo-TLD gets picked up by someone (maybe even yourselves) and becomes a real thing on the public internet.

[1] https://tools.ietf.org/html/rfc2606

I'm sorry, I don't see anything in RFC 2606 that suggests a way to choose your internal namespace not to confict with external names.

I only see a bunch of reserved TLD and second level domains, of which non seems reserved for such use.

Can you elaborate a bit?

The only way to choose your internal namespace so that it doesn't conflict with external names is to use a namespace you own, because there are no reserved domains for "internal" use.

Specifically, that's going to mean that your internal names need to be subdomains of a domain name you control, such as your organization's domain. Something along the lines of "*.internal.companyname.com".