Mail folders in Mail App iOS - Update - My Tip for solution.
The iOS Mail folders isn't "friendly " to handle.
Please see my previous question. link :
Mail folders in Mail App iOS - Apple Community
New Tip :
make 3-4 New Folders with Folder name : A-J & K-T ...
Move present folder to these new folders for easier handle .
You can also make another New Folders as needed like subject or projects.