It would be cool if there are two suika helmets. The one without glass and then you can upgrade him to the one with glass.

That will come once world features are ready! You will eventually be able to get the raw resources to be able to craft into it.

Might be better to keep Kohaku focused on combat and gathering, and leave all the glass/lead crafting and other stuff like cola and sulfa drug for Senku when you start on him. Otherwise it might feel like Kohaku's gonna do all the work herself.

Once most of the stuff is in, I can definetly consider doing something like that

Kohaku doesn't have any obvious weaknesses in the anime, so its hard to design a character that's not just a better wolfgang clone. I would say that her weakness should be based on crafting things. Make it so that she needs to build a "primitive alchemy machine" to be able to craft items with an alchemy machine (a normal one doesn't work for her). And a primitive alchemy machine is very,very costly. There's also a "primitive science machine" that she must create, which is also difficult to build. Also crafting/building items for the first time is x50 slower. In exchange, she has access to a "primitive technology" tab which lets her craft weaker versions of the original tools without needing a primitive sciencemachine/alchemy machine.