I'd do:
* SSO integration on all internal apps.
* An authenticating proxy if the org that owned it was sharp and had total institutional buy-in both from developers and from ops.
* A WireGuard VPN otherwise.
> * An authenticating proxy
I'm having trouble understanding what the fundamental difference is between these. Is it just a matter of a single, centralized proxy at the perimeter of your service network versus in-service SSO? Is there a functional difference between being in the same process space versus a sidecar on the same host versus a service on another host?
Ultimately it boils down to trusting the authority, whether that's a function (code review), a shared library (BOM), an RPC (TLS), a sidecar (kernel), or a foreign service (mTLS). There are different strengths and weaknesses for each of these, but it's not clear to me that the options you would prefer are distinctly more or less secure -- maybe there is an argument for defense in depth, but I'm not certain that's what you're pitching.