Mac Mini 2014 (Fusion Drive) Suggested OS?

FYI ... an Apple Fusion Drive consists of two physical drives: a smaller capacity SSD & a much larger capacity HDD. They are "combined" to appear as one single logical volume to the user. If your Mac mini is currently running OS X El Capitan, then that combined drive is formatted as HFS+. The Fusion Drive was designed to take advantage of the speed characteristics of the SSD along with the storage capacity of the HDD. This was at the time when SSDs were very expensive.

Ref: About Fusion Drive, a storage option for some Mac computers - Apple Support

Your 2014 mini should be compatible with macOS Monterey. However, you need to fully review your apps to see if they are as well before upgrading to the currently latest version of macOS.

Ref: macOS Monterey is compatible with these computers - Apple Support

The Fusion Drive use the SSD (Solid Drive) for the OS, i think. But i am not sure it is as fast as a real native SSD. Is that right?

Again, the Fusion Drive utilizes both a SSD and a HDD that work in tandem. The operating system is "spread" across both of those drives as it resides in the single logical volume as I mentioned before.

You should be able to find out the speed and size of your Fusion drive by going to the Apple Icon upper left corner of your screen > About This Mac > System Report > Hardware. If it is comparable to a NVMe I would say you could install anything up to Big Sur. If it is a Late 2014 even install Monterey.

My NVMe on a Late 2014 has the following speed and size:

 Capacity: 500.11 GB (500,107,862,016 bytes)

 Link Speed: 5.0 GT/s

You are welcome. My computer came to me with just a 1TB HD running at 5400 RPM, very slow, no Fusion drive. I changed that drive to a SSD then later added the NVMe as I could afford it. They are more expensive plus you need an adapter. I already had the tools from when I changed the SSD. It takes a special screwdriver to get the bottom off and work on it. You might do a search on Youtube to see about changing to a NVMe if you are interested. Just a suggestion.