Actually, I wasn't aware about the limitation of the "1 stack only".
I just thought (this whole f*** time) that you could have stacks under stacks! It felt intuitive to think this way: "Well, here is the stack option! I can place as many notebooks under this stack!... So it probably can have another stack inside! Ok, cool!" Why would I(anyone) thought that it actually can't have a stack inside another stack?! People just figures it out until they try to place a stack under another stack. But hopefully, they will try it early on, and just move it to the Tags, early on. But not me, I never had this necessity... until now, 4 years later.
The link you post is just a company telling excuses to the users that they won't do it. I think they want it to be tagged for some reason.
Well, I have no option here. I'll figure out the tags or just take this unfortunately opportunity to look at other softwares... why not.