Shameless plug (I really did not intend to promote this yet but it can help so figured I'd mention it in case you are interested), but if you are using Kubernetes you could get pretty darn close with MeTaLS[1]. Generate self-signed certs that last for 10 or more years and copy/paste them into environment variables for your service, and you've got it. Of course I don't recommend that as it's not as secure as doing things "the right" way but it's definitely better than no mTLS at all (as you pointed out).

MeTaLS won't provide you with client-related stuff, but most clients and client libs make it easy to set a certificate/key with a request.

[1]: https://github.com/FreedomBen/metals