First, on the memory. There are a number of 8 GB RDIMMs qualified on other EPYC platforms. You could obtain some through a separate channel and try them on the board and they will likely work just fine. Of course, if there is a problem with the memory, SuperMicro will rightfully tell you that you are using a non-approved DIMM and will not provide any support.
While it is not necessary to populate all 8 channels, it will be more performant than leaving some channels un-populated. You could use 4x 16GB instead, following the SuperMicro rules such that one channel of each channel pair (e.g. A/B, C/D, E/F, G/H) is populated. The degree of performance impact will be directly correlated to the degree to which your applications are sensitive to memory bandwidth. Definitely do not use 5, 6 or 7 DIMMs. Use 8 or 4.
Second on the RAID, EPYC does not include any additional hardware for RAID. You can use any operating-system-supported software RAID solution, however you would not be able to use a software RAID drive as a boot drive. Check the SuperMicro diagram for which PCI-e slots are tied to which of the EPYC socket NUMA nodes. If possible, install and configure each of your x2 RAID 1 drives onto the same node (e.g. two NVMe drives on first RAID drive on node 1 and the other two NVMe drives on the second RAID drive on node 2). The node numbers I chose are arbitrary - it doesn't matter which ones you use, just that your performance will be best if the two member drives for each RAID drive are on the same node and the two RAID drives are on different nodes.