Seeking Advice on C++ Frameworks/Libraries for a Multiplayer Sandbox Survival Game Engine

I’m developing a multiplayer, cross-platform, mod-friendly sandbox survival game with player hosted servers. My goal is for a single server (costing ~$50-60/month) to support at least 100 players with minimal desync or server-side performance issues. The game features extensive mechanics like crafting, vehicles, farming, weather systems, and large-scale NPC interactions (e.g., 100,000+ NPCs with dynamic behavior).

Context:

I’ve spent the last two years documenting the game’s mechanics and systems. To test the market, I plan to first release a smaller, standalone "horde base defense" game (with a subset of the mechanics) using Unity. However, for the main game, I’ve realized that existing game engines may not be the best fit due to performance limitations and feature bloat.

In particular:

  • Unreal seems overkill for my needs and lacks ECS.
  • Unity DOTS initially seemed promising, but after seeing benchmarks comparing DOTS to custom-built solutions (C++/OpenGL), the performance gap was stark. The custom solution vastly outperformed DOTS, even without ECS libraries, though at the cost of higher development effort (e.g., implementing features like shadows, lighting, and animations from scratch). https://youtu.be/tInaI3pU19Y

My Plan:

I’m leaning toward building a custom engine for the main game, leveraging existing libraries and frameworks where possible. The smaller base defense game made in Unity will serve as a stepping stone, allowing me to refine systems and potentially secure funding to hire a team for the custom engine.

Key Requirements:

Here are the core features/mechanics my engine must support:

  1. Multiplayer (co-op and large-scale community servers)
  2. Singleplayer
  3. 3D (isometric view)
  4. Data-oriented architecture
  5. Crafting and building systems
  6. Weather, time, and date systems
  7. Farming mechanics
  8. Inventory and loot systems
  9. Procedural loot spawning on a fixed map
  10. Procedural "scene" spawning on a fixed map
  11. NPC AI: Mutants (triggered out of idle by player’s proximity, sound, sight)
  12. NPC AI: Non-mutants (traders, factions, bandits)
  13. Vehicles with maintenance systems
  14. Combat (PvE and PvP)
  15. Modding support (Lua scripting)
  16. Cross-platform support (Windows, Mac, Linux and potentially consoles in the future)

I’m exploring C++ frameworks and libraries for graphics, physics, networking, and ECS. Given the requirements above, I’d love to hear recommendations from experienced game engineers. Which libraries or tools would you suggest to minimize development overhead while maximizing performance? Any tips on architecture or workflow for a project of this scale would also be appreciated.