Have you tried the free models on OpenCode Zen?

Models and inference engines are getting better and more optimized everyday so it is definitely worth experimenting with your GPU. You should probably check on newer finetuned and or REAP models Huggingface that could fit on your system.

I believe in your case the model quality matters a lot. To my (trust me bro) experience any MOE model, finetuned or not, with params < 19B even with Q4-quant always give lackluster results. So any Qwen2.5 wouldn't suffice to even simple coding tasks. For your case maybe you can bump it up to a Gemma4 E2B/E4B or Qwen3.5-9B. Also make sure that your current build of llamacpp actually uses your GPU with CUDA.