Hey @JKaufman hope you are doing well.
I could thinks about 2 possible scenarios here:
1) You could have a Custom object called “contracts” that will be associated to the deals you close won. The end date of this new Contract record will be one, let say 12/31/2023. Once reach the Contract end date (or 30days before) you have a WF to create a new deal for Renewal for the 100% amount of the contract (that has a calculated property with the sum of all the deal’s amount).
2) Without the custom object, You can have a custom coded action that when reached the deal A end date, search for another deal with the same end date, and create a new Renewals deal summarizing both previous deal’s amounts.
Hope this helps.