I don't have a gut feel for how much difference the Mamba arch makes to inference speed, nor how much quantisation is likely to ruin things, but as a rough comp

I don't have a gut feel for how much difference the Mamba arch makes to inference speed, nor how much quantisation is likely to ruin things, but as a rough comparison Mistral-7B at 4 bits per param is very usable on CPU. The issue with using any local models for code generation comes up with doing so in a professional context: you lose any infrastructure the provider might have for avoiding regurgitation of copyright code, so there's a legal risk there. That might not be a barrier in your context, but in my day-to-day it certainly is.