While those are important, for sure, if you are planning on multiplayer, the even more important core is a deterministic gameplay/simulation engine, to build the networking on top of.
If you’re building a game with 1500 units moving around (and can find a way to animate all of them without losing too much frame rate) then you need the network communication to not send the position of each object all the time, but instead just send player input commands per simulation tick, and make sure that all the machines resolve the simulation to the EXACT same state based on the EXACT same inputs.
All RTS-es with large unit counts use this form of networking. Unfortunately, Unreal by default doesn’t use a fixed timestep, and uses a physics engine that is not deterministic, so you can only use the built-in physics for eye candy; the “authoritative” position for each unit (which includes things like navigation and hit detection) must be simulated using a deterministic engine, and that typically should use fixed-point arithmetic rather than floating point for a variety of technical reasons. Floating point can be exact, but different CPUs may interpret the IEEE rules differently so it can’t be exact across AMD vs Intel vs ARM, and also not exact between compiler versions if you support Mac / Linux / Windows / whatever.
The best article on the things to look out for is 1500 Archers on a 28.8: Network Programming in Age of Empires and Beyond
It’s … probably 20 years ago? From an early Age of Empires game. The lessons it teaches are timeless, though – read it, study it, internalize it!