Start with a currency target on every outcome and a monthly check of whether that value was realised. Agree the baseline, assumptions, measurement period and owner, then compare the planned contributions of the delivery epics with the target. That gives you a practical way to discuss both the plan and the result.
Next, open each roadmap with Tech Debt and Bug Budget epics and allocated capacity, so maintenance is part of the planning decision. These are useful first steps towards the full model, which also retains its work hierarchy, delivery modes, approval gates and cadence.