A 64GB RAM instance on a cloud (which is what you're most likely using if you have K8S) will set you back a decent amount of money, even more so if you want one matching the specs that Stack Exchange actually uses.

If you need that level of performance you need to go bare-metal, and this is where you'll hit a lot of roadblocks (yet they will be happy to spend 10-100x more money trying to make do with the cloud).

did that before: running a single monolithic app inside a kubernetes cluster on a single pod. I still feel dirty after doing it.

My current hobby is to try and run monolithic apps like these on serverless services like cloud run. There's still some pain related to attaching persistent storage to a container but otherwise it feels like a great option.