I am doing a report on the element of Separatism in Star Wars, but have only found that the various Unions and Planets wanted to separate because of high taxes on trade routes and corruption in the Senate. Aside from Palpatine's/Darth Sidious's ulterior motives, are there any other reasons that the Separatists wanted to secede from the Republic? (Answers do not have to be canon)
In-universe de jure reasons:
As you mentioned, corruption in the Senate.
As one example, Chancellor Valorum's actions were governed by the fact that his family owned a shipping cartel.
Overall inefficiency in the Senate.
Note the respone (or rather lack of it) to Naboo crisis. Red tape, bureaucracy, squabbling.
Another concern is covered in part in the Luceno's "Tarkin" novel (new Disney canon).
It shows the process of Count Dooku trying to entice Eriadu's governor (Tarkin) to join the Separatists. The concerns (aside from abovementioned corruption and taxation) are basically that the Core worlds syphon power AND resources from the Outer Rim; and treat Outer Rim worlds as second class/subservient entities.
When Eriadu was plundered by pirates and outlaws, the Core did nothing at all to help, didn't have any Judicials' or Jedi resources. It took Tarkins' aggressiveness to take care of the issue locally.
This was directly addressed by Count Dooku (to Tarkin) when discussing the upcoming insurrection:
“I am committed to the idea of a galaxy ruled by an enlightened leader, with laws that apply universally—not one set for the Core Worlds, another for the Outer Rim worlds.”
Out of Universe reasons
Lucas, the true child of the counterculture generation, loves merchandizing (Ewoks!) when it lines his own pockets but (unsurprisingly for Hollywood) has shown deep ideological distrust in capitalism.
Note the major participants in CIS: Corporate Alliance, Trade Federation, Commerce Guild, InterGalactic Banking Clan and the Techno Union.
Basically, moneyed interests cause strife for their own benefit.
This was put into character's words by Lucas:
"I know of your treaties with the Trade Federation, the Commerce Guilds, and the others, Count. What is happening here is not government that has been bought out by business, it's business becoming government!" - Padmé Amidala to Count Dooku
In-universe de facto reasons
Lest someone thinks I made the out of universe part up, this is fully supported in canon. Several characters - from ALL walks of life (from "A New Dawn"'s conspiracy theorist and anti-The-Man Skelly - to The Man himself, Grand Moff Wilhuff Tarkin) - directly express that the whole Clone Wars in their opinion was manipulated by moneyed powers to enrich and empower themselves.
Of course, they aren't THAT far off the base, except it was the Sith (Palpatine and Dooku) who manipulated things.