Very true. He certainly accomplished much more than the majority of Sith Lords . But I'm still not sure whether there actually is a satisfactory answer to my question. I'm still in two minds as to whether he did accomplish his goal and it just seems there are some flaws in his plan.
Actually, he accomplished much more than any Sith Lord. His only remote competition is Darth Revan, Darth Bane, and arguably Darth Krayt. But, I understand your question: since he ultimately lost it all, did he really accomplish his goal?
He did establish an effective government after the corruption of the Republic (lets face it, Dictatorships can be effective, however unjust), and a military force to match and kept the galaxy at relative piece for years. However, he only really destabilised the Jedi Order, and despite his best efforts, a number of survivors slipped through his net.
Palpatine didn't actually care about the remaining Jedi prior to Order 66. Rise of Darth Vader makes it clear that they pose no real threat to him. And they didn't; not militaristically, at any rate. They were all scattered with precious few means to contact each other. Only to satisfy Vader and to test his power did he allow Vader to continue to hunt the Jedi.
You would think that given his nature and intelligence, his well thought out scheming and plotting, that such a big factor in his victory would not have been prone to any such failure. But no, a number of Jedi slip through the net, including two of the most powerful Masters of the Council. Is this a character flaw? Is he so arrogant that he wouldn't consider Ben and Yoda, or more so, their teachings dangerous in the long run?
One could interpret it as arrogance, but he was right in the long run: militaristically, no Jedi posed a threat to him. Hell, not even Luke was a threat insomuch as he wasn't dangerous to Palpatine by himself. It was his connection to Vader that made him a threat. The Jedi were scattered, vastly outgunned, and - remember - Palpatine assumed the twins died with Padme.
Even if those facts were overlooked in his arrogant nature (a trait all Sith have seemingly invoked afterwards) then why, considering his knowledge of Anakin's relationship with Padme and how he used it against Anakin in his fall, would he not search for the Skywalker offspring? (Bear in mind, I'm not up to date on these poorly written EU novels and subsequently, he may have done)
Rise of Darth Vader makes it clear that Palpatine thought that they were dead.
If there was anything that would destabilise his power, it would be a Skywalker, and since Anakin was all dark and brooding these days, maybe he just presumed it would still mean a Sith rule. But surely, given his knowledgehe would consider the possible Skywalker line a threat and would make sure of any heirs?
Palpatine did not want heirs. He intentionally designed the Empire's infrastructure so that it would not last without him, thus displaying the depth of his selfishness. That is why no Imperial usurper or warlord could come close to bringing the Empire back to its former glory. Hell, not even Thrawn could do it.
Vader's relationship with Palpatine was complex. He wanted to overthrow him, but he also admitted to Luke in RotJ that he "must obey his master", thus indicating he obviously had a frightening mental stranglehold on Vader to the point that he had to make sure Luke was ready to kill the Emperor before he could attempt it.
I am still in two minds about this. He did control the galaxy, he did finish the Jedi Order as an institution and he kept his apprentice in check, and he could also laud it up around Coruscant with a rocket launcher, reaping the rewards of insurance; but did he plant the seeds of his own demise through a character flaw or through a lack of foresight? Or is it just dramatic neccesity that he overlooked these things?
I consider it more or less "fate", but Palpatine helped the process along. Pre-RotJ sources revealed that Palpatine was aware of Vader's treacherous thoughts. The thing was: like most people who have thoughts of murdering another human, Palpatine thought Vader didn't have the balls to do it, deep as he was in the dark side and Palpatine's manipulations. The trap at Endor worked brilliantly. Even with the unforseen Ewok assistance on the Sanctuary Moon, Palpatine could have owned the Rebel Fleet at Endor had he not ordered the Imperial Fleet to simply keep them from escaping. Or, at the very least, he could have escaped.
But the reason he couldn't do this is because the Force was starting to balance itself once again, as explained by Palpatine being unable to sense Luke's presence.