Why would you want 8mm z threaded rods? They may be slightly straighter but ussually not by much. They are stiffer and will more forcefully wobble the nozzle compared to 5mm.
If it's wobble you need to eliminate, there's a much easier way to do that, and eliminate z nut backlash in the process.
I used this anti wobble part to decouple the xy shifts and works well:
anti backlash thingiverse
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Kind regards
Imqqmi
NFAN CoreXY printer:
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reprap.org]
