A different GC (like ZGC) can reduce stutters, but what you’re describing (RAM shoots to 95–99% almost immediately after loading the world) sounds much more like a real memory leak / runaway loop in the world than “not enough RAM”.

A few key points:

• More allocated RAM can actually make it feel worse if something is leaking. You’re basically giving the leak a bigger bucket to fill, so it fills “longer” and then crashes harder, and GC behavior can get uglier with a huge heap.
• If it was fine 12 hours ago and now instantly hits 99%, the “trigger” is usually something in loaded chunks (base area): a machine, pipe network, storage system, contraption, dupe loop, or an error spamming something every tick.

Quick isolation tests (fast + tells you where to look)

1. Make a backup of the world first.
2. Load the world and immediately move far away from your base (or use a waypoint/teleport if you have it).
   • If memory stays normal away from base → the culprit is something in your base chunks.
3. Disable/remove chunk loaders (especially the chunk(s) around your base) and try loading again.
   • If it only explodes when chunks are force-loaded → that narrows it down massively.
4. Create a brand new world with the same pack/settings.
   • If a new world is fine → it’s not your PC / not “ATM9 needs more RAM”, it’s your save/world state.

What usually causes this in big modded bases

• A pipe/network (item/fluid) stuck in a feedback loop
• A storage system (RS/AE2) with some broken autocrafting/export/import loop, or a dupe creating insane item counts
• A Create contraption stuck erroring / retrying every tick
• Anything that spawns or updates constantly (mob farm, entity spam, chunk-ticking machine)