Just my opinion, based on the experience I'm going for within my server (difficult and semi-realistic):

I think it should be around 80. If it's above 80, then the weight reduction needs to be nerfed accordingly because 100% is too much. That option menu is definitely needed. Also the currently crafting requirements are also too lenient, it should requite a higher carpentry level and more wood.

I personally will use when building stuff, those 40kg anvil + 40kg bellows are really annoying to deal so having 80 with 100% would be perfect.

As for real balance it would be best to be lower around 35kg for a improvised manually built wooden wheelbarrel seems good, but having such a low amount would only work if more options are added like metal carts and trolleys.

Or maybe you build that simple wheelbarrow with 35kg and then upgrade it, use a car wheel and it adds 5kg, replace with metal bars and it adds 10kg, replace wood planks with metal sheets and it adds 20kg.

80 with 97% weight reduction