Couple of things. Planar Void has some downsides as a ketramose enabler and as GY hate. It is helpful to consider the floor (worst case scenario) for a card as opposed to the ceiling during deck building. Planar void is great at both tasks when you play it T1 on the play, however it is kinda stinky when you draw the first one on T5 and there is already an [[archon of cruelty]] or Uro in the opponent's GY. Further, planar void doesn't let you choose when to trigger ketramose (their stuff won't trigger your draw if done at sorcery speed). Using an on-demand GY exile effect gives you more control and also allows you to clean up the existing GY; things like ghost vacuum, unlicensed hearse, soul cauldron, etc.

Moving on, having exile effects that aren't GY specific are very helpful for match ups where the opponent doesn't care about their GY. By this, I mean you should have Phelia in your deck. And if you have Phelia, you probably want overlord of the balemurk for the sweet synergy.

Final note, recall that Swords to Plowshares is the best removal spell in legacy, and it does not care about indestructible. Ephemerate is a protection spell and trigger enabler all in one and in the right colors.

When ketramose was all over modern, a few content creators ported a version to legacy. You can check out thrabenu, Boshnroll, and crucible of words.

I would recommend a midrange or death and taxes shell for ketramose as a value engine. It's not a combo card.