When composing a Mail message, you can change your selected font and font-size using the format button. While your cursor is within the body text, tap the aA button that you’ll see at the top-left of the on-screen keyboard:
Here you can select the current font - and adjust the font-size, font colour, justification etc.
As for why the native Mail App doesn’t visibly indicate the selected font size (presumably a point-size), only the Apple Developers will know.
Apple does invite submission of comments and feature requests via its Product Feedback portal. For iPad and iPadOS:
Feedback - iPad - Apple
If you would like to see enhanced or additional features in a future version of iPadOS, or associated Apple Apps, this would be the most appropriate channel through which to communicate your request to Apple.
Turning to final question, you can install an alternative third-party Mail App, of your choice, on any compatible devices. After adding your email account(s) to the third-party Mail Apps on each device, they should remain synchronised.