The RAID uses disks individually, so the cache in the disk controller should be unused (or irrelevant compared to individual disk cache), so it wouldn’t help to have more RAM there.
What would help is a larger + separate NVMe (preferable with low latency) for use as RAID cache. You can also configure it to use RAM as RAID cache in the advanced settings.
For speeding up deletion (and reading) the RAID metadata disks help, but I see you already went this way: Error adding RAID Metadata device . Often when it cleans up it needs to read file system metadata first (which causes the iowait if that is not in cache).