Sorry for the long post, just thought I'd share a solution that's working for my needs. Maybe it might work for yours or someone else looking for something similar.
About 2.5 months ago I was looking for a NAS solution, but I didn't want to spend alot of money on it.
I was looking for a way to add more storage space and some form of data protection/ backup to my PC.
I have looked at various software like windows storage spaces, freenas, unraid, xpenology for example.
Since I only have 1 PC, that I need to use for work, games, plex server, radarr, sonarr and other stuff. I needed something very versatile.
As I was looking for different options, I came across a program, StableBit Drivepool.
Drivepool creates a virtual hard drive that combines all hard drives that you configure to be part of the pool. So you can have 1 BIG drive. It also offers various options on managing data on the physical drives themselves; Like setting space thresholds, spreading data across the multiple drives or not, data/ space balancing, duplication, etc. Without having to reformat or lose existing data on the drives.
They also have DriveScanner program that monitors hard drive health. It can integrate with Drivepool, so if it detects a faulty hard drive it will automatically start moving the data off that hard drive.
Another cool product of theirs is CloudDrive. If you have a supported cloud drive account/s you can configure them in CloudDrive and it will show up on your PC as a hard drive on windows explorer.
And no, I don't work for StableBit.

I just decided to try their software, liked it, bought it, now sharing my experience of it.
Anyways, I've got 6 drives in my PC now; 1x Samsung SSD for windows 10 pro, 1x 1TB WD Black drive for Plex program data, and frequently changing data, 4x 3TB WD Red drives for media storage, infrequently changing data.
Now that my SATA ports are all used, next would be to either upgrade to bigger drives or add SAS HBA card with pass through to connect more drives.
Oh, 1 more thing, depending on your needs and type of data you are storing. snapRAID does parity calculations on your drives/ data that has been configured. It does need 1 dedicated hard drive that should be the biggest. So if a hard drive fails, it can rebuild the data from the parity stored on the other drives. Only thing is that it doesn't work well with some of DrivePool's balancing features, so it is recommended to turn off those options.
I have been running this setup without a problem for about 1.5 months now. Even did windows 10 1903 update, no problems.
As for Plex streaming, I use nvidia shield tv. I did notice that some files would transcode on my PC when I use the normal Plex app, but if I use Kodi Plex addon, then it doesn't transcode.
Anyways, just thought I'd share. Hope you find a solution that works for you. Good luck.