That’s fair, I think this may work, but even if it does, I don’t believe this is a problem.
Given that the end-of-window time (which is bounded by the current time) goes up by P per block, it means the real-time block production rate in your scheme is exactly P per window, exactly the intended rate. So, from the perspective of validating nodes, there is no resource consumption attack.
And beyond that, indeed, this allows miners to reduce the difficulty while keeping the block rate constant. But a colluding group of miners is always able to do that: they can just agree to all reduce their hashrate, for any difficulty reduction they’d like. But, all that achieves is making it easier/cheaper for a competing miner who doesn’t follow the scheme to take over.