Hi Daniel,
Based on the number of players, and if that means that a lot of the map may not be visited by one of the players during a match/playthrough - then Lightweight actors could be a good idea to reduce the numbers of static actors as well (this also saves memory). There is a good community article here: https://dev.epicgames.com/community/learning/tutorials/zweW/light-weight-instances-ue5.
For geometry collections as well, there are some tuning parameters in GeometryCollectionPhysicsProxy.cpp, specifically
bool bCreateGTParticleForChildren - this is set as true normally, if you set it to false it won’t actually make the child bodies until a destruction event occurs.
For collisions, favour implicit geometry where possible (ie Spheres, Cubes etc), keep Mesh geometry as low as gameplay will sensibly allow - avoid long thin triangles, tiny triangles or huge triangles.
Insights and Chaos Visual Debugger are the primary tools I’d use in this case, but if there is anything specific you are looking for I can find out.
Geoff