is it bumpsteer or need of rnp?

I believe you can go to a specialty alignment shop and they can measure to see if you have bump steer issues.
If you do , you can usually iron them out with a bump steer kit.
Baer makes one, and pro motorsports make one

Bumpsteer is basically when the car steers when the suspension cycles. The There really isn't any harshness necessarily like you describe like crashing into a curb. Though the lower the car is made is worse the problem becomes.
There is most likley some issue other than just bumpsteer.
I would check your strut rods, bushings etc... It is there job to absorb the rearward force of a bump on your suspension so that it moves up and down, rather than front to back. Something may be going on there

Also
I have a Randalls Rack, and it won't improve bumpster that is inherent in the mustang suspension since the geometry doesn't change it is basically duplicated. That said the road feel, response and drivability are greatly improved with a rack and pinion. but you may have some other issue that is affecting your suspension.

I can't really comment on the steeriods.

If bumpsteer is the only issue then a bumpsteer kit can help. They lower and move the tie rod ends forward on the spindle. This 'also quickens your steering ratio since the arm on the spindle is now a shorter lever.

Hope this helps.

this sounds very similar to a problem i was having, turned out to be a combo of a few things......... #1 the bushing in thebrand new idler was way too soft... ended up going to roller bearing set up. #2 was the shock tower flex.... mine was abnormally extreme amounts...cured by a brace. the brace led to a few other issues. (will find the next weakest place to flex) #3 lowered about 2", 245/35/17's with aggresive alignment. big tires+ race spec aligment=bad highway driving(catches ruts)

i ended up deciding to go sn65 style...... i just got sick of the constant "issues" and lack of driving enjoyment during "normal" conditions.

good luck