You need to understand couple of things:
1) to create a sap connection and not using smart integration connector you can either use the brapi or create connection using erpconnect directly
https://helpcenter.theobald-software.com/erpconnect/documentation/sap-connection/log-on-to-sap/#connect-to-sap
The error you get is because you are trying to use the classic rfc protocol which requires that library to be in the onestream servers. Therefore support are the only that can fix that.
2) when you upgrade to 8.2/8.4 you will have to use SIC as erpconnect is not available in the platform and most likely your sap in only accesible via private network. In this case the BRAPI to create the SAP connection is not available anymore and you will have to create the connection in a SIC Function BR as shown in the link
Being said that, first thing id try is to set the NWRFC protocol in you sap connection object. If you set the NET WEAVER protocol it will not use the classic rfc library. I don’t think that will work because that requires other libraries which most likely are not in the server.
can you attach here screenshot of your BR properties (referenced assemblies) and the BR code?