It’s not that simple.
If you want CSFB you will need a 2nd SDR and to run the Osmocom 2G stack, then configure both this network and the 4G one so that fall-back works. You will have two separate stacks, complete with their own base stations and core networks.
If you want VoLTE you will need a 4G network and to configure an IMS. When doing this people tend to use Open5GS for the EPC and then Kamailio for the IMS. But there are some more details to take care of and you can find these online.
If you want to do both, you will need both of the above. In other words:
A functioning 2G network with its own BTS/TRX, BSC and core network.
A functioning 4G network with its own EPC/core plus IMS.
A 2G core is different to a 4G core. You need both and as far as I know you will need to provision subscribers in both.
This is nothing to do with SDR really and a question of mobile network architecture and the questions would be much the same if use COTS non-SDR equipment. The best place to ask questions is the Osmocom and srsRAN etc. communities. Once the SDR is providing network service it is no longer an SDR matter and rather instead one of RAN/Core/IMS configuration.