It started it small increments and then sometimes was abandoned. I'd say the first Doom solo story that didn't have him coming up with some plot against the FF or a plan of world conquest was probably Astonishing Tales #8 by Gerry Conway. That story from the early 1970's was the seed that inspired Roger Stern's GN Doctor Strange/Doctor Doom : Triumph and Torment.
Conway would later write another Doom appearance in his Sub-Mariner run that may have been intended in the Astonishing Tales series. This was a time when Marvel has these shared titles. Astonishing Tales featured Doom and Ka-Zar but Doom's solo came to an abrupt stop after issue #8 and then it was just Ka-Zar. But issue #8's last page teased another Doom story that never appeared
Gerry Conway would become the writer of the Sub-Mariner solo title ( #40-49) for a while and perhaps only coincidentally he titles the first issue of a 3 part story "Deathmasque" with Doom and Namor teaming up. In this story Doom seeks the Cosmic Cube but not for the reason Sub-Mariner suspects. Doom does not want to use the Cube for world conquest but he saw it as a chance to transform himself so that he was no longer scarred. I have volume 6 of the Marvel Masterworks HC collection of the Sub-Mariner series which includes his 10 issue run from #40-49 (#39 was Roy Thomas's last issue and is included here). Conway wrote an interesting forward to this in which he describes going to the local candy story with his mother when he was 9 years old and the cover of Fantastic Four #4, which featured Sub-Mariner's debut into the MU caught his eye. Ten years later when he was 19 Stan hired him to write for Marvel Comics. Six months later, he would be writing the Sub-Mariner series. He describes this Sub-Mariner/Doom tale "a partially successful exploration of his (Namor's) ongoing friendship/rivalry with Marvel's other hero-villain Doctor Victor von Doom" So he would be one of the first of the new generation of Marvel writers to look at Doom that way, something we see in the earlier Astonishing Tales story.
Thanks! I didn't know they created that Twitter account.