There is no question, you must amend your 2018 and 2019 tax returns to include Form 8606 to reduce the taxable amounts of your RMDs for those years and to reduce the amount of your basis carried forward from each of those years. Even if you do not amend to claim a refund, you must reduce the amount of basis carried forward as if you had completed the 2018 and 2019 Forms 8606 properly for each of those years. The tax code does not permit you to pick and choose which years you apply the basis.
