A low tech and quite effective way to do vias is via micro rivets. They’re popular for CNC’d PCBs, e.g.: https://wegstr.com/riveting-set-for-PCB-vias-0.4-mm?search=R... (No endorsement, just for the pics)
https://www.vpcinc.com/Category/Favorit-Through-Hole-Rivets-...