Hi ya there Peter,
You are doing really well for your first time planking and although you feel it is a struggle when you sit back and look at it - it isn't that difficult. You are feeling it is a struggle because it is a whole new field to you. You seem to be worrying about using the polyester resin but first things are first - about tapering planks, the way you have gone is the easiest method and there is no right/wrong way of planking a hull JUST THE BEST WAY FOR YOU to achieve the job, which you are doing and really successfully. Now, there are a lot of fillers which can be used, but the thing to consider is if some of the fillers become damp - will they degrade and come adrift? that is the only thing to think of when you are choosing the filler, if you are confident that you are going to seal the hull perfectly, then the choice is wide open to you. You don't have to use water resistant fillers such as P38.
When you come to the time to actually seal the exterior of the hull, then we can discuss the variety of sealers such as polyester resins and epoxy resins plus there is another product called a G4 which might be easier for you to use and therefore are happier with. Until you reach that stage though, carry on with your planking, you are making a very good job of it.
aye,
John