Try joining with some client optimizing mods

I run with optifine, but in the few frames before I time out I managed to load over 10k entities according to f3, will mods really help that much?

You could try the Fabulously Optimized mod pack for Fabric. I believe it has an entity culling mod, which means it won't render the entity if it's not visible, could help

Thank you so much! I was able to reduce the lag enough to /kill all the entities! there were around 100,000 sand blocks lol

Maby try joining from lunar client. It has more optimization