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There's KSM that should help: https://pve.proxmox.com/wiki/Kernel_Samepage_Merging_(KSM)

Probably works best running VMs with the same kernel and software version.

But that just seems to reintroduce the same problem again:

> However, while KSM can reduce memory usage, it also comes with some security risks, as it can expose VMs to side-channel attacks. ...

It will! For Linux hosts and Linux guests, if you use virtio and memory ballooning.

This was an issue for me a few years ago running docker on macOS. macOS required you to allocate memory to docker ahead of time, whereas Windows/Hyper-V was able to use memory ballooning in WSL2

It's possible to address this to some extent with ballooning memory drivers, etc.