Homebrew Campaign

I'm currently in a campaign as a player and we are nearing the conclusion. I will most likely be taking over as DM next campaign and was looking at doing a homebrew campaign. I have what I feel would be a solid start but how do you go about preparing for session after or would it be prudent to wait to see where the players go and plan from there and essentially go session to session?

For a homebrew campaign, it is important to have multiple tiers of enemy planned and some vague idea how players will transition from one of these enemies to another. For instance:

1) the campaign starts with fighting Sahaugin, Merrow, and Pirates in a coastal / sea-faring adventure. During this part they find / come across a treasure map that leads them to the next part.

2) the campaign turns into a survival / dungeon crawl through dangerous tropical forest to a lost temple, through the monster-infested & trapped temple to find the pirate treasure. Removing the treasure from the temple unleashes a Lich / Curse that become the villains for the 3rd part.

3) The Lich/Curse release an army of Demons that start spreading across the land and the curse blocks out the sun. The party can go on side quests and ignore this threat but eventually the curse will catch up to them and they will have to assault the demon/undead-filled fortress of the Lich with its many magical traps.

4) Destroying / defeating the Curse reveals that some evil god is behind the whole thing, and now the party have to prepare to fight an actual god...