Improve slow cold start of HDD with some sort of cache?

Hi together,
My problem: I've got an HDD (two in a mdadm-RAID1 in fact) that goes to "deep sleep" after not being accessed for a while which I find wonderful. It does not make any sounds anymore and certainly saves energy.
BUT: An access to this HDD is followed by a freezing/loading of the file explorers of about 8 seconds in which the HDD starts up. Of course while the HDD is started up the access speed is fine.

Now what I would love is to have a portion of my RAM or SSD (about 1GB) act as a write cache and as a read cache only for the index. I only want to have to wait for the startup time if I actually read file data or write more than the cache can fit.
Priority one is to have the full file index cached; priority two is to have that write cache. A read cache is not necessary because to me being able to quickly find and select data to copy is the important part. After initiating the copy / read access I can just do something else.

I could not find anything that specific but I have to admit I don't know what exactly to search for. I tried "cold start hdd cache" and similar queries but to no avail. I do not really have much experience with caching. But after many years with these HDDs it still does grind my gears a bit each time I have to wait for the startup...

Note: I have noted that many times the most recent accessed directory listings seem to be cached in that the HDD keeps being turned off while I re-list some directories. Only when accessing previously unlisted directories the HDD begins to start up. I assume this is some sort of default linux cache? Or even a cache inside my HDDs?

I am using Arch linux and the HDD-RAID is connected via USB 3.0 if that matters.