The problem - to make a mule character with fully unlocked stash / bags you need to play quite a bit and spend lots of iron.
The solution - let players make a mule character. Just add a checkbox ‘Mule character’ so when you create a character with such option checked you get a toon with fully unlocked stash tabs, bags. But !! this characters can’t wear any items. Simple. And can solve the problem of infinite ‘stash’ storage
Solution, install GDStash, craft some inventory bags and put them in the transfer stash then take out however many you need, drop them on the ground and pick them up to make them appear.
Look, your suggestion is as valid as any of mine.
But mules in single player are an obsolete design imho. I muled in PoE for quite a while until it essentially became unbearable (and finally admitted to myself that I was going to play the game a lot!). But PoE is not moddable.
Here you do have storage options with search features. And the install is very simple. It further has no impact on your drop RNG and overall luck.
Instead of making the developers spend precious time and resources coding a feature for the niche subset of players that insists on hording to an extreme but also in avoiding third party tools to an extreme? Medea that’s an outrageous idea! Outrageous I say!
Solution: Organize transfer.gst files in your folders and label them accordingly. No need for mules. Transfer stash files can be your infinite stash. That’s what I do.
I still just use multiple .gsh files
Just always seemed the easiest solution.
Hey, I don’t do outrageous very often. Deal with it! 
Or even better - why use the stash? Just take GDStash and craft any item with any stats you want lol 
I just thought of a way for people like me who doesn’t like using 3rd party software in games.
then wait for FG, I believe the token that unlocks eg Elite also unlocks the inventory bags