I decided to late to get the maxle upgrade on my Five when I bought it.
Is there any way to stiffen up the rear end ?

I was kinda thinking something like this

Superstar thingy

they are sold out for now, but would something like this work.

DT Swiss skewer maybe? Some people reckon they make it stiffer. Or get a 9/10mm bolt through one.

If it’s a Pro2 hub you could swap to a bolt-up axle. That stiffens things.
If your using a Shimano hub you could always swap the axle to a solid one, with nuts.

I had the Superstar one on my Heckler and it made a noticable difference.

Bolt up/thru will help a bit. But even the maxle’d Five is still one of the least stiff rear ends in class. (my 224’s rear end is pretty much as stiff as my titanium hardtail!)

Embrace the bendy! Everything you lose in directness, you gain in grip. Think of it as sideways travel.