In general, performance RAID and SSD are not a thing you need to do. The performance improvement is pretty minimal, and you'd only want to use RAID as a way to maintain multiple drives in case one begins to fail, allowing the other drives in the cluster to handle the load. If you're talking 10+ drives, performance RAID can come back into play.
HDD simply wont improve your performance compared to SSD. An SSD will r/w faster than 3 HDD in RAID 0. You can't beat the abandonment of mechanical parts in an SSD.
I recommend using a calculator like this one to get an idea of your I/O speed results vs. requirements. Dedicated RAID vs onboard RAID is pretty much identical as far as performance and reliability.
In all cases, SSDs do indeed drop performance over time, but with modern SSDs, TRIM-style operations prolong this to a point that it won't slow down to anything close to HDD speeds until the drive is on its last legs, which is roughly two years after an HDD would have died anyway.
If capacity or cost are less of an issue, and all you want is performance, then a RAM drive may be the way to go. These are faster than SSD and not subject to the same wear, and if you're doing that much work at a time, this might be a better approach. You'd still want periodic backup to permanent media, but RAM drives are pretty much designed for how you described your use-case.