If you need a lot of local storage indeed HDD + SSD caching with Primocache is the way to go.
But it does not substitute a boot SSD, better if NVMe.
I'm using SSD caching long before PrimoCache was even released and I never "exhausted" anyone so far.
Of course there's wear but the cost/performance ratio is always incredibly high.
Most of the normal operations on HDDs are SSD-like speed.
If you have multiple M.2 slots go for SSD caching with a cheap M.2 NVMe; I bought an Intel Pro 7600p 256GB on Ebay, about 30 euro.
Just consider that you need a lot of RAM; I have a cache size of 8 GB, 32KB block size, 1.90 GB of overhead.
You can use a bigger SSD but for a normal usage it doesn't really help and the overhead will be higher.
You need at least 32GB system RAM to make it worthwhile.
Smaller block size and get faster but also more CPU usage and more overhead.
For a normal usage I found 32KB to be the most convenient.
Usually 90-99% hit ratio on HDDs and 50-60% on the boot NVMe.
I have an M.2 970 Pro for boot, you can exclude the L2 caching and use the same cache task; no waste of RAM for dedicated caching.
The RAM only caching is awesome on SSDs and far better than Samsung Rapid mode and others.
Still good even with 8GB test size:
I also have an horribly slow Samsung 860 QVO SATA; I use L2 caching on it and it's pretty darn fast too of course.
And there as well awesome performances with 8GB test size:
Copying big files on this SSD can be slower than on a HDD; with Primocache it feels like using the 7600p NVMe drive.
I've never regretted purchasing Primocache, highly recommended.