A seller over the threshold registers through INBiz (or the Streamlined Sales Tax Registration System) and collects and remits Indiana sales tax as the State's agent. The official material read does not set a grace period between meeting the threshold and the start of collection, so the prudent reading is that registration and collection are due once the threshold is met; confirm the start date with the state before relying on any delay.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.