Nickname and Channel services are almost entirely a non-anti-unstandard in IRC, with the only exception being SASL auth, and pseudo-standard PASS auth, and defacto-standard /NICKSERV auth. You will never find an acceptable IsAuth solution until a standard is both proposed and adopted, which has not happened.
I agree that you cannot hope for mIRC to recognise every authentication method on every IRCD ever written.
My request is limited only
to those authentication methods already recognised by mIRC by offering authentication in the server definition - and only
to delaying autojoin to channels until it receives some response to the authentication request or a timeout expires. I think this is possible.