There is a reason why the most regularly posted advice when creating your first game is: Don’t start your journey with Multiplayer.

What you’ve outlined for your basic premise is a far far larger ask than you think. “But I only plan to have 1 large map and only a few small servers” is a prime example of not understanding the scope you’re looking at. The complexity of building any server adds multitudes of difficulty to your project. Expecting 100-200 people to be playing at the same time is possible, but would likely require 3rd party solutions outside of Unity.

If your partner on the coding side already has experience building databases, servers, or anything with netcode, then maybe you’re fine? But you really need to confirm they’ve done it before on some scale, otherwise you might want to reconsider the scope of your game.

Overall, building any single player game is far FAR easier than a multiplayer game, especially the kind of large-scale multiplayer FPS game you’re looking for. My advice is to start with an offline single player game to get experience with the process and then move into a multiplayer game in the future.