Very pleased to see that you decided to go for an 8mm tube👍
That leaves you some 'meat' to play with for bushes/bearings😊
If you use a 6mm running tube with a 4mm drive shaft, as some have advocated, that only leaves you 1mm to play with for outer tube and bearings. Not a lot☹️ Resulting in a comparatively flimsy arrangement that I would only want to use for very short shafts at low revs. If at all.
Personally I would regard 3mm as the absolute max diameter of a shaft in a 6mm outer, and then only for short e.g. >/=6" or so shafts.
That's my humble twopenn'orth anyway.
Wonder whose shafts / bearings will last longest 😁
Cheers, Doug 😎