That's a temporal ontology called atemporalism - it's not as obviously incompatible with free will as foreknowledge omnitemporalism (God knows the future so it must already exist), but it does have the same problem. If God sits outside the timeline, comprehending it as a whole, the timeline, as a whole, must 'exist' for God - He knows what we'll do next year because He's 'already' there; it still implies only one possible future. In this view the universe is a classical 4D Parminidean 'block' (you could try playing with the idea of God existing over
all possible timelines, but a moment's thought shows it only makes the problem worse, whether only one timeline is actually realised, or all of them). There is the further problem of how an atemporal God can act temporally, i.e. act
within the timeline...
Omniscience also has a problem for God Himself - for His actions to have moral value, He must deliberately chose the good. If He knows every action He will take, that deliberate choice doesn't seem possible (if He
always knows
everything, He knows without deliberation), which also implies a lack of free will..