Yeah, Doom has never really had his own solo series. The best you'll find is the above mentiond mini. Outside of that, his character really is one that has been expanded on over time due to so many appearances in other stories.
Off the top of my head i'd suggest Doomwar (Doom makes war against Wakanda), Emperor Doom (Doom conquers the planet), Doomquest (Doom and Iron Man are sent back in time to the era of King Arthur and Morgan le Fay), the original Secret Wars (Where Doom gained the power of the Beyonder towards the end), and Dr. Strange and Dr. Doom: Triumph and Torment (Probably my favorite, where Strange and Doom travel to Hell to rescue the soul of Doom's mother).
Doomquest also has 2 sequels in the form of the Recurring Knightmare storyline, in which Doom and Iron Man go to the future instead and once again meet Merlin and King Arthur, and Legacy of Doom, where Doom and Iron Man go to hell and fight over Excalibur. Last time I checked you can get the first sequel and the original story in one trade paperback volume that's simply titled Doomquest.
I suppose there's also Doom 2099, which would actually count as a solo series I suppose. Have yet to read it though, so I can't suggest it, but i've only ever heard good things about it.