You won’t get close to SSD speeds no matter how you set up HDDs (well, unless you give them a cache). That said, the impact on your application performance is going to be against your IO time only. If you don’t do a lot of IO (because you have so much memory that nothing really hits the disk) then this could be fine. I find that if you build it (a database with fast storage) then they (IO intensive SQL) will come, so you’ve probably already made this a requirement going forwards.
In reality, you may be trying to solve a problem that doesn’t exist - are you really going to wear out your SSDs faster than you can afford? The network attached storage idea with RAID is fine as a general storage solution, but you’re going to want SSDs if your application is already relying on your database being supported by SSD.