Here is the parts diagram for your front forks & everything else come to that if you navigate around. The parts prices are eyewatering, so take care if you decide to modify anything.
https://www.motorcyclespareparts.eu/yamaha-parts/motorcycles/xt1200ze-2015/front-fork-735424.aspx
I'm not sure this really helps unfortunately & I am wondering, having changed the rear spring, whether you might be creating your own problem here.
The ES/ZE bikes are sprung a tad lighter than the standard Z model but, given the 'on the fly' adjustments the package, they seem to work pretty well out of the box, just as long as you are within that 205Kg/450lb max loading. If you need firmer than one helmet solo, just stop & add luggage or even 2 helmets & you will change the ride dramatically.
Personally, I am around 200lbs clad - I run with the standard preload settings & a Std or slightly soft damping set up, largely due to our poor roads here in the UK. The bike rides/tracks/handles a treat. Get on a fast smooth stretch & it's so easy just to firm up the damping if needed. Still I guess set-up is always a very 'personal' thing.
Maybe someone with experience of changing the front springs can assist. Several have changed the rear, as you know, but like I said earlier I can't recall any feedback about the front end.
Here's hoping you can find a solution .........................
KEN