When you say quite laggy, please provide a little more context. Is it lag in the graphics? Are you seeing lots of beach balls while waiting for things to happen?, etc.
Which processor does your Mac mini have?
What size SSD does it have, and how much free space is available?
Are you using the machine in the same state that it was in when the previous owner sold it to you?
Was the machine reset when you bought it so that it was like setting up a new machine out of the box?
8GB of ram should be plenty for the vast majority of users when using a machine with a modern ssd as long as it has ample room for swap. Unless you are doing some serious work in photo/video editing or using it as a gaming device, you are likely fine with the ram as is.
If your machine only has a 128GB ssd, I would recommend only having macOS and the apps stored on the internal ssd. Then I would attach an external ssd and use that to save my files. Those little ssds fill up quick and can bog down a machine when they do. With 256GB or larger, you can keep more files on the internal drive and back things up externally as needed.
As far as which version of macOS to use, I would use the newest version that supports the hardware. OS upgrades do not bog down machines with ssds like they did in the days of spinning hard drives. Unless the choice is driven by a piece of software requiring and older version of macOS or you prefer to stay a revision or two behind the latest available in hopes of greater stability, stick with the newest version for your machine.
