A7V production actually started in early 1917 - but proved to be a very slow process, because tank building didn't receive any priority - until Cambrai happened.
The trick for the POD is to find someone 'in the higher echelons', who sponsors tank construction.
I've done this by 'recruiting' Max Bauer, who IOTL was the most powerful enemy of tank construction (until mid-1918, when Saul turned into Paul). And I employed Willy Rohr, the father of the assault battalions, to come up with the tactics for tank employment.
Blairwitch has Bruno von Mudra, who soon is to become Chief of Staff of the German Army, as main sponsor. We'll see whom he picks for tactical evolution.
Given such PODs, a powerful German tank force of 200 to 1,500 vehicles could become ready for the 1918 spring offensive. The number would depend on the type chosen, heavy or light, or a mix of both.