The simplest way to improve a process is to analyse the errors that it lets slip through.
For every error not found by one of the previous quality control processes, ask, is there a strategy that could have been applied to one of the existing models that would have created a test case that could have identified the error.
If the answer is yes then it may well have slipped through because timescales or staffing levels forced your hand and you simply didn't have the time to apply that strategy to that model, or the risk of not applying it was deemed low.
If the answer is no then we have to identify a model and a strategy that could have found it.
In both cases assess the cost and time impact of adopting that model and strategy. The development process is one of trade offs and compromises.