The problem has nothing to do with SRP in itself, but that the Firebird .NET provider version 5.0.0.0 only added SRP support, but does not yet implement the wire protocol encryption. This wire protocol encryption does – by default – depend on SRP for its session key, but the fact SRP is implemented, does not imply that wire protocol encryption is implemented.
Firebird 3 by default requires encryption, but as this is not supported in Firebird .NET provider version 6 and earlier, you get the error “Incompatible wire encryption levels requested on client and server”.
To be able to use wire encryption, you need to update to Firebird ADO.net provider version 7. Version 7 added support for wire encryption, see ADO.NET provider 7.0.0.0 for Firebird is ready for more information.
Or, alternatively, you need to configure Firebird 3 to enable but not require encryption by editing firebird.conf:
WireCrypt = Enabled
And then restarting Firebird.