If you're familiar with the Linux boot process, you may be aware that the system often does run from RAM for a period of time, before mounting a filesystem from a block device and calling pivot_root to transfer the rootfs over.
If you want to run the OS from RAM, the absolute simplest way to do it is to simply never transfer. Building a custom initramfs has never been easier. There are tools explicitly geared towards making an initramfs, like Dracut, but also a huge ecosystem of tools for building container images which could almost certainly be scripted to spit out a cpio without a lot of trouble. You can even use something like Buildroot.
As far as loading it from the network easily, I would also look into Unified Kernel Images (UKIs). They combine the EFI stub, the kernel, and the initramfs into one single file. You should be able to load that directly from the PXE firmware.
If you want to run the OS from RAM, the absolute simplest way to do it is to simply never transfer. Building a custom initramfs has never been easier. There are tools explicitly geared towards making an initramfs, like Dracut, but also a huge ecosystem of tools for building container images which could almost certainly be scripted to spit out a cpio without a lot of trouble. You can even use something like Buildroot.
As far as loading it from the network easily, I would also look into Unified Kernel Images (UKIs). They combine the EFI stub, the kernel, and the initramfs into one single file. You should be able to load that directly from the PXE firmware.