Well, this stuff is just broken. The docs say:
https://www.devart.com/dotconnect/mysql/docs/?Devart.Data.MySql~Devart.Common.SshAuthenticationType.html
"Default Auth Plugin" as the fragment for the connectionstring to specify the auth plugin, right? Well....
Exception type: NotSupportedException
Keyword not supported: 'default auth plugin'.
Although this shouldn't really be needed as the docs also say:
"This option may be omitted because the client may receive the name of the authentication plugin when performing handshake, but you may use this option to shorten client/server negotiation and avoid a round trip in the protocol."
but omitting it gives the error you saw too.
So using 'standard' it works, using 'cached_sha2_password' it doesn't connect. I'll file a bug report with devart. Hopefully they've fixed it in a few months.
It sucks that the mysql ado.net alternatives all suck one way or the other. The vanilla one from mysql is so bad I don't even want to look at it (plus it's GPL licensed, so not an option). There's a promising OSS one but it lacks features we need (schemas etc.) so not an option at the moment and there's devart, which works if the moon is in the right quarter.