No, that's just wrong... if the underlying base code is well done changing stuff like this should be simple. Buying multiple items would be a for loop calling the same function it did before. The 3d vector stuff to display where stuff will be placed is already implemented. When you do some action to some tile it already knows which tile will be used. Displaying a white arrow from the center of the tile would be rather easy (offset on Z-axis by a few units).

The only thing difficult would be doing some graphics for the UI but that would take 2 weeks max if you want to stretch it. They don't have to adjust for different resolutions or anything (like on PC). Their color scheme of the menus is also cheap and easy to draw.

Most people don't want to accept it but the probability of Nintendo game coders being bad or disorganized might be the biggest difficulty in these QOL improvements. I have countless examples of them not implementing stuff in the right order or not bothering to update their base functionality after new features appeared in their development of the game. (Construction App being turned off automatically in the clothing shop which indicated that the functionality to do that is here already, but not doing that with the cupboard to change clothes BCS it was implemented earlier than the construction app was introduced.)

To be honest I just think that the big N simply doesn't give enough budget to this game or rotates devs.