I agree with Kotor fanatic. Mandalorian War was for weakening Republic. TS didn't have enough firepower to conquer Galaxy without weakening Republic and Jedi Order first.
So, my theory:
At the end of The Great Hyperspace War (propably biggest and most devastating war between jedi and the sith when it comes to numbers of jedis and siths and battles. Galactic civil war was the ultimate and last confrontation between these two factions that ended to defeat of the sith, but there was only one jedi, Luke, and 2 siths...) when end of Sith Empire drew near, Ludo Kressh, Naga Sadow's old rival, sent forces loyal to him to destroy Sadow's remaining fleet. The two Dark Lords met up, and Kressh banished Sadow from the Sith Empire forever. As the fleet of Empress Teta arrived, Sadow purposefully crashed one of his ships into Kressh's flagship, ending Ludo's short-lived reign as Dark Lord of the Sith.
However, remains of Kressh's fleet vanished into deeb space, never heard again... Here we have these mysterious True Sith.
Slowly (there is 1000 time span between Great Hyperspace War and JCW) True Sith's grew stronger in unknown regions building their Empire and plotting - well they were siths after all - against each other and Republic (especially jedi).
It has to be remebered however, that Reoublic was far from weak in military power before Mandalorains wars, most of the forces was just dismissed, as heard from Carth Onasi, and Republic was caught by suprise.
TS's force had become formidable, but as Republic had numerous and strong jedi order with them AND strong military, direct attack from unknown regions would've been catastrophic defeat or at least too risky. So better to weaken Republic first.
I am sure TS's knew about Exar Kun and his war due their many spies, and mandalorians effort in it. Which other group would suit better to weaken Republic than this vicious warrior people? And during this (as TS's surely knew that mandalorians would be defeated in the end) they would erase this possible threat after they would've conquered whole Galaxy.
So as we know, mandalorian wars began. Jedi Order did not come to it thoughm as they sensed greater threat beyond Mandalorian wars. Especially Vandar sensed this. This threat was naturally True Sith. After Revan and renegade jedi started to help Republic in it's war efforts, tide of war turned leading eventually destruction of Mandalorians.
The part I aren't sure is, did TS cause Revan's downfall and JCW. Final battle could've been in other hugely important (for mandalorians) system than Malachor V. But maybe TS knew that Malachor V was more important than any other system for mandalorians, and maybe they knew that Revan knew it too.
So maybe TS planned Revan's fall, maybe not, it isn't main question. BUt let's presume that they planned it (or atleast hoped for it).
JCW began, and after events we know, this led to pyrrhic victory of Republic. They might actually be worried when Revan triumphed from one fight to another, but I believe that no matter what Revan would've done with protecting Republic's infrastructure, no war can strenghten nation (in this case Galaxy) so unknowing it by him/herself, Revan aided TS's greatly.
When Malak became to Dark Lord and started his brutal war, TS propably jumped up and down for joy, As Malak preferred an all-out war of destruction and carnage. It was this part of war, not Revan's elegant (but weakening anyway) tactics and methods of conversion and assasination, that ultimately nearly destroyed whole Galaxy.
In the end Republic emerged victorious, but as I said before, it was Pyrrhic's victory.
"The Jedi Civil War destroyed the Jedi. By the war's end, barely a hundred Jedi remained. Many fell in battle... and many more were seduced by Revan's teachings."
This was said by Kreia as you all propably remember. Republic was in ruins. Now it was time to strike for TS.
First year after Battle of Rakata Prime went propably in gathering forces and such, so that's why they didn't attack directly after war's end. After year passed, Revan vanished, remembering what he had learned about True Sith and their plans.
Before he went to UR, he left T3-M4 and Ebon Hawk behind, to search help in his struggle. And he left Canderous behind to unify mandalorians so they could help in coming war. And as light side, he left Bastila and Carth to "keep Republic strong". Well, this leads to Kotor 2 eventually.
Then comes the question? Why they didn't attack now. I agree with Jediphile in this matter.
Because Revan somehow foiled their plans, and managed to hold TS's inevitable attack. How?
After all, Revan is one person, so he couldn't simply destroy True Sith. One man against propably at least millions? Right... Revan was tactical genious and mastermind.
TS's are sith, always fighting for power. Revan created civil war between factions of True Sith, and in this way prevented (or better word should be held back, as no civil war is never ending. Ultimately one will win and unify the TS) attack. Surely (s)he has some plan, but he needed help in that too. Exile is his first "helper", but propably not the last. Aslightside Lost Jedi re-establishes Jedi Order and as DS Disciple/Handmaiden with remaining hiding jedi (as Nihilus is gone) does the same thing.
Events of Kotor 2 are not directly connected to True Sith threat, since Nihilus was threat to anything that lives.
So this part was its own threat, not connected with TS, as Jedi order was annihilated in JCW already. 100 jedi doesn't do much good.
And what happens next, will TS manage to finally laucn their doomday kind of attack? What happens to everything, siths, jedis, Revan, Exile, Republic? This all will be told in Kotor 3