I assume, when you talk about Meraki auth not being secure, you mean MAC Authentication Bypass (MAB)? I can see why you would say that it's not greatly secure, (this is not the same as Meraki auth BTW)
What other options were you expecting? As we are authenticating at Layer-2 on a switch - in order to effectively open the port - options are bound to be limited.
You mentioned you also have MX - it may be possible to police access at that level - maybe something like this: https://documentation.meraki.com/MX/Content_Filtering_and_Threat_Protection/Configuring_Active_Direc...
But your switch ports would be open access, if that was all you were using. If you're trying to block client devices 'at source' then switch acces policies are the way, but the options in the document I linked before are the only ones available.