@dbuero ncp-disable directive disables “negotiable crypto parameters” for the Data Channel. Therefore, the client becomes free to pick a cipher for the Data Channel in the cipher directive, if such cipher is supported on the server side.
Without ncp-disable it's expected that you get the error you mention, except for the 1st "favorite" cipher the VPN server wants to negotiate (AES-256-GCM).
That's also the reason why we say that AES-GCM has priority over CBC: the CG will generate "cipher AES-256-CBC" for backward compatibility with those OpenVPN old versions that don't support ncp and/or AES-GCM, but newer OpenVPN versions will negotiate AES-GCM. cipher none is not available in any case, as our servers are not configured to admit unencrypted connections for obvious reasons.
Anyway, unless your system is loaded by other tasks, a single i7 core (which implements AES New Instructions) can perfectly handle your peak bandwidth, especially when AES-NI is supported by the system. If you see anyway that a core is at capacity (it can happen if your system is loaded by other tasks), try our suggestion:
ncp-disable
cipher AES-128-GCM
Don't bother with CHACHA20-POLY1305 if your system supports AES-NI.
Kind regards