How does developing another engine help at all? It is very likely its graphics are so under-optimized because they spent most of their time on the simulation aspect, which is more than challenging enough. Dyson Sphere Program is a game with lots of stuff going on at a time, the engine isn't the issue. Cities Skylines 2 is just a very complex game.

As a city builder it has not just dynamic geometry (which makes graphics optimization more challenging) and lots going on on screen like DSP, but also a complex agent based simulation of a city, its transportation, its economy, and individual agent AIs. It is far more complex than most games despite not being graphically intensive. And do note that as far as cities builders go, it's also one of the most ambitious graphically. My personal opinion is they deserve some slack as long as the game continues to improve.