35B-A3B means it's a MoE model with 35B total parameters but with only 3B active at once. The one I use is the 27B dense model. Usually, dense models give better responses, but are slower than the MoE. With your 4090, you should be able to get about 50 tok/s with the dense model, which is more than enough for practical use.

If you want to keep using the same model, these settings worked for me.

llama-server -ngl 99 -c 262144 -fa on --cache-type-k q4_0 --cache-type-v q4_0 --host 0.0.0.0 --sleep-idle-seconds 300 -m Qwen3.5-35B-A3B-UD-Q4_K_XL.gguf

For the harness, I use pi (https://pi.dev/). And sometimes, I use the Roo Code plugin for VS Code. (https://roocode.com/)

I prefer simplicity in my tooling, so I can understand them easier. But you might have better luck with other harnesses.