An often overlooked, but very solid system was the one presented in the original Mercenary's Handbook. It is still available officially via PDF for 5-10 USD. Well worth the price, if you do not have it already.
Attached below are two, very old, personal systems that I have used with some success.
The first is really the Operational Game from the Tactical Handbook, cleaned up and made to work with a modified system that used BF2 stats for individual units (not unlike AlphaStrike today). While the TacHandbook took a lot of heat in its day, the Operational Game was and remains solid. It has been so long since I've looked at it, though, I will assume much of the text is taken from the source material so I cannot but assume the credit should ultimately go to the original authors.
The second file was a bit more successful. The first half deals with how to play the aforementioned TurboTech. Ignore that and skip to the campaign section and you may find something useful.
Then there is an Excel file with some spreadsheets designed to help the systems previously described. Along with a sample that would be handed to a player so they can build a force. There would have been another one (the companion to the file here would have been for a Kuritan force).
Take your time and skim them. They have been pretty heavily play-tested and work well, though as a GM there is an initial amount of work that needs to be done before the campaign can pretty much run itself.