There's a bunch of pretty simple, old products for using SSD's to provided caching for higher latency drives that are all about ~a decade old. Some of them even use Linux's device manager or the block device subsystem explicitly...

https://github.com/mingzhao/dm-cache

https://man7.org/linux/man-pages/man7/lvmcache.7.html

https://www.kernel.org/doc/Documentation/bcache.txt

https://github.com/facebookarchive/flashcache

https://github.com/stec-inc/EnhanceIO

Doing RAID-1 (or RAID-10) style solution certainly is reasonable, but I don't understand what's novel here.

...and IIRC this is the kind of thing that btrfs and zfs were managing explicitly...