Sure you can use the build-in ssd cache in QNAPs version of MDRaid. But notice that QNAP will make the SSD cache part of the filesystem, so if your cache fails your entire raid might fail, or at least you can loose all write data in cache. QNAP recommends at least two SSD drives for cache.
The other point is that, and we don’t know this yet - the spread of reading files. Let’s say all your data is read evenly, that means your cache is inefficient and would not improve things, most likely the other way is it has to serve the file, and at the same time write it to SSD cache, where it would never be used again.
Now I’m not saying it will be this way, but atm it looks like most are using the network for a second backup copy, i.e. write once, read zero.