Just get some House and Garden or Canna coco, its pre buffered ready to use straight out the bag no fucking around . Mix it 70/30 with perlite and its right to plant.
There's no way you can have enough K to fully hit you potential in a soil at the start of a cycle that won't prevent the proper uptake of other nutrients most importantly calcium, properly timed top dress may be able to do it but you'd have to be monitoring leaf sap which I don't know of anyone...
Without an analysis you're only guessing but I'd say you could but you wouldn't be getting the best quality or yield out of your plants. Water only soils in my opinion are just as much a gimmick as the bottled nutrients they are always banging on about, why deprive a plant of the shit it needs...