My home network mostly runs on 10Gbit ethernet cables, of which the unraid is even connected to 2 NICs using aggregation of the 2.

My unraid server has 32GB of DDR4 in it, of which it barely ever uses more than a third. Which surprised me, but that's another topic.

So; I'd prefer my RAM to be always used to its max, and therefore I'd like to either have a RAM-disk set to (for example) 20GB of the free RAM left at all times, or have the OS arrange that maxing it out automatically for me. I'd say having RAM be used for I/O of data-streams is highly preferred over using the SSD storage, while unraid's slackware does not seem to do much caching in RAM. I've been monitoring the server's RAM usage with netdata over a month.

I support this, I have 196GB's of RAM and I noticed that after a copying for a bit the speed decreases massively. My system has a UPS on it, that can keep it powered up for 30 mins so losing data to power loss is a non issue for me.

Same here, UPS and a very reliable 220V mains network too. Barely ever any outages that were not planned or announced beforehand.

 

There are things you can set in /etc/sysctl.conf to make more use of RAM, but I did that already, and even then it still uses only around 12 GB max.

This is a linux issue in general, I've noticed. The distros all claim they cache and use tmpfs, all the while in reality you barely see the RAM having much I/O over time. As soon as you manually create and mount a RAMdisk, it suddenly changes. So I'm guessing that's also the case with slackware and unraid;

You have to create a ramdisk with a static size and name, then have unraid use that as its preferred cache storage medium.

Take a look at the Tips and Tweaks Plugin. You can play with settings there to influence how Unraid handles its RAM and when it writes to disk. Definitely read the thread for suggestions.