For rts games, i recommend the spring-engine. Its kind of date visually, but the core multiplayer tech is rock solid.

There is a fresh fork developed by the team behind https://www.beyondallreason.info/ and they are working on performance improvements. In addition, you get the whole infrastructure developed by that team for free (Server tech, matchmaking, map editors etc.)

Its hyper specialized on RTS though. Dont try to make a jump and run with it. Although they tried: https://www.youtube.com/watch?v=a1D295mRqdE https://www.youtube.com/watch?v=kNGe4KTdVN4 https://www.youtube.com/watch?v=-FnmQUgR5RQ

Ludum Dares are pretty hard core engine tests, as in can you iterrate, pivot and check wether its fun quickly. Unwieldy beasts do not survive that.