Sure you can use a PE with a declining ratio over the years,...

Sure you can use a PE with a declining ratio over the years, however this approach is highly unorthodox and relies on arbitrary inputs; the PE ratio you choose for a given year.
In a DCF valuation the arbitrary number is the discount rate, however much less so as it is a widely used number across the market and relevant to lending rates, market conditions etc.
The DCF valuation was just an exercise to put the current trading price into perspective in light of some highly speculative and optimistic valuations; it is a futile, and often frustrating exercise to harbour expectations which the market will unlikely ever fulfil.
However the fundamentals are great for this company and full valuation will be achieved once they’ve proven themselves capable of delivering consistently on the BFS assumptions.