Overall notes - I would recommend working via Line Items on the deal record (adding, updating, etc.), then creating a quote. (Do not recommend creating a quote, then adding or updating line items during the quote creation process.)
First part - HubSpot does not allow for custom billing terms outside of what is in the Billing Frequency drop down. What I have done is switch to Billing Frequency Monthly w/ Term (Months) 18 but add a personalization token of the TCV into the comments section of the quote with formatting to make it extremely noticable (not beautiful but, until HubSpot allows customization on quote total source, the only way I have found). This is tricky because line items on the deal must be updated prior to quote creation for this personalization token to update correctly as changes are made.
Second part - I would add one line item for a full year of the new module cost (ex: $12) w/ Billing Frequency Annually and one line item for a one-time discount given in this particular year (ex: $4) w/ Billing Frequency One-Time. This will make the quote total equal to what the customer needs to pay ($8) to add the module this year, but ensure ARR is correct for future years. (Important not to add discounts and fees on the quote creation page because they will not calculate into Revenue Field updates on the deal record).
Falls ich weiter erklaeren kann, lass mich bitte wissen! 