We first tested 100 player cap, it was running smoothly.
They then increased to a 500 player cap with 3 DGS per shard. I was on shard 170. It crashed shortly after reaching max capacity and the RMQ network tech was struggling with an Interaction delay of roughly 40 seconds (+ high ping and desync along with it). Moved to a fresh shard soon after. Shard 090, which handled 500 players a lot better (about 2-3 seconds interaction delay).
Testing then moved on to a config with 1000 players per shard and 6 DGS per shard. First few shards all got 30k very quickly after hitting max player count. They reduced a shard (010) to 750 cap. I was on 010. It managed to not crash for quite some time. Server fps was still very good but as expected, the RMQ was still struggling hard with an interaction delay between 45 sec and 1min30 on average, which prooves again that the issue now isn't meshing itself or server amount, but on the RMQ tech to prevent that interaction delay when exchanging data between all the servers and clients.
Now as I'm writing this post, player cap has been reduced to 600 and servers are holding well. Server FPS consistently at 30 but interaction delay remains at 40 seconds on average.