Hi @DCrinnion1
You are right in that ARR is only set by HubSpot and it is calculated as far as i know based on amounts and term length / billing frequency etc. as captured on line items associated to the deal. So if you were able to create a line item with the correct start and end dates, including adding additional line items for multi-year subscriptions, then theoretically ARR/MRR should update accordingly.
Exactly how you would get this to work alongside the new “Subscriptions” module that I believe you are using in the eCommerce hub I’m not sure. But I know that with line items, you could for example have a period with start and end dates that are discounted - e.g. for 1 year, followed by another line item that starts at the completion of the discounted period in the full amount, etc.
Getting ARR/MRR reporting accurate can be notoriously tricky, especially if you also need to manage upgrades, downgrades, churn etc. Sometimes Operations Hub Pro/Ent is required with custom coded workflows, custom objects etc.