While creating a nation-wide integrated radar and AD system at the scale that
@Kirkhill envisions might be impractical (i.e. short and medium range local detection vice the long range, wide area detection systems of NORAD), perhaps a reasonable approach would be to install a series of passive receivers at key strategic locations across the country and deploy mobile transmitters (manned AEW&C, UAVs, Fighters, mobile ground based radars, etc.) as the threat conditions require. Gives you a multi-static system when required without the requirement to have a massive number of high-power transmitters permanently deployed where there is typically nothing of significance to detect.
Same with AD systems. Rather than trying to have an AD/C-UAS unit assigned to cover every possible target in the country, deploy batteries from centralized units as required. And here I'll agree that we probably need considerably more AD units than just 4th Artillery Regiment (GS), both for domestic and deployed operations.