wimlib

Wimlib limits LZMS multi-threading to 128 MB chunks due to high RAM use per thread. Larger chunks force single-thread to avoid crashes. Virtual memory isn’t used because swapping hurts performance. This is by design, not a bug. For multi-threading with big chunks, use LZX or keep chunks at 128 MB or ...

You’re right that mspack expects the decoder to reset Huffman tables each frame. Wimlib’s LZX code assumes a continuous stream, so it doesn’t reset tables per frame by default. Fixing this means deep changes in how wimlib handles Huffman state across blocks.
There’s no special LZX block to reset ...

Yeah, I’ve run into similar limits when trying to get better compression within Windows, you really hit a wall when it comes to integrating anything beyond what Microsoft officially supports. I tried playing around with WIMBoot setups years back, and although it was clever, it wasn’t really seamless ...