I lost the trash icon in my mail app iOS

Are you sure that you don’t have the email account on the Pro set to Archive mail instead of Deleting with the Trash Icon? IIRC, several years ago, Gmail users were saying that they couldn’t delete emails and it was because they had their accounts set for archiving in the settings and the trash icon wasn’t appearing.

https://9to5mac.com/2018/12/19/gmail-delete-instead-of-archive-iphone/

TBH, I've never had a trash can icon next to the edit button. It's not there on any of the four email accounts I have on my iPad. And I doubt it's something you can just restore on your iPad. The mail app on your iPad is not configurable. It is the way Apple made it work on your iPad. Perhaps your iPad Air is running an out of date iOS version now?

You can provide feedback to Apple directly that this is something you'd like via this link: Feedback - iPad - Apple

They won't respond to you, nor does saying want something mean you'll get what you want, but it's the only official way to let Apple know you'd like to see what you want.

When you are in your mailbox and you choose edit, do you not see little circles next to each email? You tap each circle you want to highlight for deletion, then the trash icon should be on the bottom. Tap the trash can, then confirm you want to delete the mail you selected.

I must not have been clear. Different mailboxes to me means like Gmail on the Air, and Yahoo on the Pro. If you are on the same iOS version on both devices, and you are using the same mail accounts, they should look the same on both devices.

I have a gmail account that I use as one of my secondary email accounts. I rarely use it and haven’t had it all that long. I do remember though, that when I first starting coming around in ASC years ago, some Gmail users weren’t able to delete emails because they didn’t have the trash icon. Their accounts were set up to archive the emails.

What do you mean running completely different mailboxes?

It's been a while since I had a non-Pro iPad. Perhaps it's the difference between a Pro and an non-Pro.

At this point, I'd recommend you reach out to Apple Support.

Before ever responding I went to mail settings to see if this is any way user customizable and it's not.

Sorry, I couldn't solve this for you, if it even can be solved.

I'm glad you came along. I'm not a Gmail user and am not familiar with it. I hope this is the OP's issue.