Synology SSD cache size requirement

I've been running a new Synology RS3621RPxs for several weeks, and my 4TB SSD cache shows: Cache composition: Reusable (341 GB) / Total (3.6 TB) If I'm reading that correctly, my nas isn't even using 10% of the available SSD cache. According to the Synology website this was the right size for my array, but it seems like overkilL! My cache hit rate is around 85% The NAS is used primarily as an NFS data store for other hosts using ESXi. (So large VMDK files primarily). This SSD is a RO cache (single SSD), and I'm thinking of switching to a pair of NVMe sticks for RW cache. But, what size should I buy? Does RW cache increase demands? I hate to spend the money on 2x4TB of cache if most will go unused!