I think you’ll end up with a nice and fast setup doing that provided your controller can keep up(older controllers were generally built assuming they’d never see something as fast as an SSD, so they can have surprising bottlenecks).
The question is whether your speed/cost/size/app performance trade off is optimal? Also whether you care about optimal? And then how much you are willing to invest in time for experimentation? When it comes to disk space for me, whatever number I think I can live with in trade for speed eventually becomes too little. Whatever tolerance I think I have for failure also understates the pain of recovery from failure that happens a lot with disks, though more so with spinning, at the worst possible time.
I have to qualify all that with the observation that getting to a caching and redundancy setup that I am satisfied with has been a long process that really isn’t complete. I have learned that you can produce a high performance setup with fast nvme and/or SSD as cache, backed by a redundant array for size and security.
In terms of absolute dollars to produce the headline 2GB/s round number you are headed for, it will likely cost as much if not a little more. It wold potentially have significantly more space (12 spinning 2T or 4T with 2 nvmes for cache wold be 20 or 40 TB respectively with 2-drive fault tolerance).
2T disks are not going to produce per unit throughput as high as 4 which won’t be as high as 8 of course… but 150is a decent round number per unit (reflecting the difference between inner and outer tracks and 2T vs 8T rates). I do know 10-12 HDD in raidz2, particularly with compression enabled can deliver 1GB/s+ throughput. So your sustained sequential is that with nvme front end delivering much more for cached entries and write.
Once you’ve gone past 4 drives, you can start to do some nifty things with such arrays.