the britpart Orange shock with the yellow bumpers I was able to get the bottom nut on after compressing over night.
But the top nut ?
To compress the bumper & also screw on the nut, Nada no way.

is there any better way to do this?

I assume the bumpers you are referring to are the bushes? I do not use poly shock bushes for this reason. even the oem black rubber ones have the same issue. What I tend to do is fit the lower bush (shock side), and then tighten it down with a spacer, then fit the upper and again tighten it down without the washer, then hopefully having tightened it down a couple of times things have seated into place and compressed enough to be able to fit the washer and get the nut started! I have never not had one fit using this method but i have had several that have required lots of swearing at them before they finally relented and allowed themselves to be bolted up.

I was about to pick up a mall realized I had to stop, figure out a better way.
The poly bushes may be great if they will stay compressed whilst parts are shifted, unlikely.
a thinner washer sounds like best idea tho.
& assuming there will be several go arounds to get there.
Thnx

I share your pain ... lots of swearing when I did mine too.
I think the threaded sections are shorter than they used to be, I can't remember having this issue last time I changed shocks (it was a good few years ago), an extra 5mm and it would be simple.
I use the @dag019 method too.

You sorta have five to ten nano seconds to get the final top cap in place & get the nut started.

I had left the assembly compressed over couple of nights, plus using a flat washer for compression.

Same here, ended up using thinner washers to get the nut started, after much messing about and failing.