Planetary system architectures in the final stages of planet formation

The processes in the last stages of planet formation determine the final assembly and composition of planets. After the protoplanetary discs dissipate, a system of planets and planetesimals -- analogues of the Asteroid and Kuiper Belts -- remains. Ongoing collisions between these bodies produce second-generation dust and gas that form a debris disc. Resolved images of such belts allow us to trace the dynamical history of these systems, including giant impacts and planet-belt interactions, during the crucial era from 10 to a few 100 Myr of terrestrial planet formation. High-resolution mm and optical/near-IR imagers have revolutionised our view of the brightest planetesimal belts, providing first glimpses of diverse structures, many of which show plausible planet imprints, while ALMA has revealed gas in a growing number of systems. Yet these results are strongly biased toward the brightest, mostly young belts around early-type stars, and even for these, crucial details remain undetectable at the required resolution due to their typically low surface brightness. A future (sub-)mm facility with an order of magnitude leap in continuum sensitivity and 10x higher line sensitivity would transform our understanding of late-stage planet formation by enabling the first statistically meaningful surveys of faint outer planetesimal belts across all spectral types, imaging planetary collisions in the inner regions of planetary systems, and gas measurements that trace volatile delivery to terrestrial planets. For the first time, this would reach the faint majority of belts, rather than only the rare bright discs accessible today.