904 with very hard 1-2 shift

I agree with the others on checking the servo spring first. One simple mod to firm up the 1-2 shift is to remove the servo spring and install a blocker rod; if that was done in your 904 along with a shift kit like a TransGo TF-2 or TF-3 the shifts will be extremely firm. I installed a TF-2 shift kit in my 904 and did the transfer plate mods (drilled holes) for the firmest shifts but I put the 1-2 servo spring back in when I reinstalled the valve body which is what the TransGo instructions call for. It shifts pretty damn hard even under light throttle at low speed, borderline harsh maybe for some but I like it. I can't imagine how it would be with a servo blocker rod in there, I'd be afraid of breaking something in the driveline.

Another thing to consider, the stall speed and "looseness" of your torque converter in combination with final drive (rear end) gearing will have a noticeable effect on how the impulse from a hard shift is transferred through the driveline and ultimately how it is felt by the driver (and passengers). A loose converter and taller rear gears will reduce the felt impact from a hard shift.

EDIT: Also worth a try checking the line pressure adjustment first, there is a spec either for number of turns of the screw or distance of the "screw nut plate" from the VB, can't remember which.