Well, your view as to a “friendly UI” may differ from others. Speaking personally, accepting that you may have an alternative opinion, I find the Mail UI to be both intuitive and flexible.
If you are experiencing difficulties with your iPad, perhaps you’ve not yet discovered the iPad User Guide?
The iPad User Guide is a rich source of frequently overlooked information about your iPad and iPadOS. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892
The Tips App, installed by default on all new models of iPad, provides a brief introduction - and leads you to the User Guide. In case you missed it, here is a link to the Tips App:
https://apps.apple.com/gb/app/tips/id1069509450
Apple also has a huge YouTube Channel - containing videos and feature guidance:
https://www.youtube.com/user/Apple