Can I back up my entire iPad contents externally then download on to my new iPad?

You can backup your iPad to a computer, and then use that backup to setup your new iPad.

Or you can set up your new iPad from an iCloud backup made directly from your old iPad.

See the following links.

click here ➜ Back up your iPhone, iPad, or iPod touch in iTunes on PC - Apple Support

click here ➜ How to back up your iPhone or iPad with iCloud - Apple Support

click here ➜ Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

click here ➜ Set up your iPhone or iPad - Apple Support

The process to transfer data from old to new iPad is straightforward.

These support pages should get you started with setup of your new iPad - and the methods of transferring your data from old to new iPad:

Set up your iPhone, iPad or iPod touch – Apple Support

Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

Use Quick Start to transfer data to a new iPhone, iPad or iPod touch – Apple Support

If the donor iPad, from which you are transferring your data precedes iOS12.4, QuickStart will not be available.

“Transfer” is rather a misnomer. In reality, your data and setup is merely cloned to the new device. Nothing is moved or removed from the original device.

An alternative is to restore either an iCloud or iTunes backup to the new device:

How to back up your iPhone, iPad and iPod touch – Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Be aware that Apps are not directly transferred between devices. For Apps, the transfer process creates a record of the Apps that are installed on the donor iPad - and it is this record of installed Apps that is transferred to the target (new) device. For Apps that are still available for download from the App Store, the corresponding Apps are automatically downloaded and installed from the App Store.