What needs to be done before the final mission?(spoilers)

Anything on the Hyperion needs to be finished.

I shot rayes lol

Ryder Family Secrets
High Noon - Kadara main story
Dissension in the Ranks - Allies and Relationships, final stage of questline that starts with "Know Your Enemy" on Voeld
Cora Harper: At Duty's Edge - loyalty mission
Jaal Ama Darav: Flesh and Blood - loyalty mission
Peebee: A Mysterious Remnant Signal - loyalty mission
Nakmor Drack: A Future For Our People - loyalty mission
Turian Ark: Lost but Not Forgotten - Allies and Relationships, final stage of turian ark questline
Water Supply - Elaaden local assignment, starts from a terminal in the Flophouse
Uncovering the Past - Voeld local assignment, final stage of a questline that starts with "Stage a Rescue" on Voeld

Full completion of Ryder Family Secrets unlocks a different scene in the space battle over Meridian, where Ryder figures out how to use the Remnant and the Scourge to avoid kett fire.

The person in charge of Kadara at the end of High Noon, Reyes or Sloane, will come to Meridian to shoot up some kett and provide assistance. Of all the people that can come to help Ryder, this person is probably most prominent. If you do not complete High Noon, Sloane is there by default.

Completing Dissension in the Ranks allows you to choose whether to strike a deal with the Primus, one of the Archon's subordinates. Striking the deal means you fight fewer kett on the way to the Archon and changes a post-ending cutscene. You make the decision on the deal during the final mission, not during Dissension in the Ranks.

Successfully locating the asari ark (Cora's loyalty mission) and the turian ark (Lost but Not Forgotten) changes things in the middle of the battle for Meridian. Captain Dunn is forced to crash the Hyperion on the surface after ordering all remaining crew back into stasis. If the asari, turian, and salarian Pathfinders are not all alive and well, Dunn dies; otherwise, she survives. All you have to do in Cora's LM is choose between Sarissa and Vederia; either Pathfinder works. Same with the salarian Pathfinder from the crit path mission Hunting the Archon; either Raeka or Hayjer suffices. But in Lost but Not Forgotten, you need to actively convince Avitus to become the turian Pathfinder, not just complete the mission. Otherwise, there is no turian Pathfinder, only asari and salarian ones, and Dunn dies.

Completing Jaal's LM means his family appears to help out in the firefight at the chasm in Meridian. It doesn't matter if you kill Akksul or not. (Not killing Akksul does mean Jaal gets a lovely scar for the rest of the game. He looks very dashing.)

Saving Kalinda during Peebee's LM means she also shows up in the fight for Meridian. If she dies, there is no replacement.

Completing Drack's LM allegedly means Vorn can show up in the battle for Meridian. I, uh, didn't actually see this happen, but maybe I just had too many people helping out.

Taking Annea's bribe at the end of Water Supply means that she joins the fight for Meridian; failing to complete the mission, or refusing her bribe, means she doesn't have a replacement. Also, refusing her bribe means you get attacked by her mercenaries on Elaaden.

Refusing to kill the angaran AI in Uncovering the Past, and then giving the AI to the angara rather than having it work with SAM on the Hyperion, means the angara allegedly get AI-improved gear during the Meridian battle. Haven't tested this either.

The two different salarian Pathfinders have different dialogue and whatnot during the rest of the game depending on your choice in Hunting the Archon. In addition, saving the krogan scouts (i.e. opting for Hayjer instead of Raeka) means that the krogan help you out during the last battle before the final boss, and also means that you don't have to fight a Behemoth in that last push.