I was right where you are when I worked on my quarter panel armor. I had enough riv-nuts to set that I wanted something better. I bought one of those cheap Chinesium tools. It worked. Sort of. It does not provide great control for keeping the riv-nuts centered as they compress. I suspect the battle is against burrs on the back side of the hole. If you could deburr the back side things would automatically be straight. But if you had access to deburr the back side you probably would not be using a riv-nut in the first place. As a consequence, some of my riv-nuts ended up being not quite centered in the holes they need to be. I think I wound up drilling several out so I could do them over again.
Then a few years back I needed to set some longer riv-nuts (had to go through plywood in the walls of my cargo trailer). The Chinesium tool did not offer long enough mandrels to manage the longer riv-nuts. I broke down and bought a pneumatic tool as that was the only thing I could find to do that job. The pneumatic tool is far superior. Of course, it cost far more.
The tool I have is an "Air Pull-Setter" from Hanson Rivet and Supply Co. One of the benefits is that the mandrel part of the tool is simply a standard Allen head capscrew. That means replacements for the most likely worn component are available at your local hardware store, and you can easily adapt it to a different length riv-nut with a different length capscrew (again, readily available). On the down side, the mandrel driver is specifically sized to that Allen head (in this case 5/16") so I can't switch over to a differently threaded riv-nut (in this case 1/4-20) (actually, 5/16" being about equal to 6mm I could probably run M6 riv-nuts if I wanted, but that doesn't make for much of a difference).
I have not looked into trying to convert the pneumatic tool over to a different size, like say 3/8-16. I suspect it would be substantially more work than what is required to swap sizes on the cheap Chinesium tool. That would be why the pneumatic tools are sold as a size specific tool.
All that said, the pneumatic tool is far superior. It provides more control for putting the rivets in such that they are kept straight. If you have developed the muscle memory to keep a hand drill straight then you already have the motor skills in place to keep this tool straight. If I had a bunch of riv-nuts to install in a different thread pitch I would seriously consider buying another pneumatic tool.