I have a similar requirement, I need to get invoice information after invoice save from admin.
I fulfil the requirement using the event observer.

app/code/Anshu/Customization/etc/adminhtml/events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="controller_action_postdispatch_sales_order_invoice_save">
        <observer name="observer_admin_invoice_save" instance="Anshu\Customization\Observer\AnshuInvoiceSave" />
    </event>
</config>

app/code/Anshu/Customization/Observer/AdminInvoiceSave.php

<?php

namespace Anshu\Customization\Observer;

use Magento\Framework\Event\ObserverInterface;

class AnshuInvoiceSave implements ObserverInterface 
{
    /**
    * @var \Magento\Framework\Registry
    */

   protected $_registry;
    
    public function __construct(
        \Magento\Framework\Registry $registry
    )
    {
        $this->_registry = $registry;
    }

    public function execute(\Magento\Framework\Event\Observer $observer) 
    {
        $invoice = $this->getInvoiceObject();
        // My Customization
    }
    
    private function getInvoiceObject()
    {
        return $this->_registry->registry('current_invoice');
    }

} 

Check if it is helpful to you.
Magento version was 2.2.1

I have a similar requirement, I need to get invoice information after invoice save from admin.
I fulfil the requirement using the event observer.
app/code/Anshu/Customization/etc/adminhtml/events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="controller_action_postdispatch_sales_order_invoice_save">
        <observer name="observer_admin_invoice_save" instance="Anshu\Customization\Observer\AnshuInvoiceSave" />
    </event>
</config>

app/code/Anshu/Customization/Observer/AdminInvoiceSave.php

<?php

namespace Anshu\Customization\Observer;

use Magento\Framework\Event\ObserverInterface;

class AnshuInvoiceSave implements ObserverInterface 
{
    /**
    * @var \Magento\Framework\Registry
    */

   protected $_registry;
    
    public function __construct(
        \Magento\Framework\Registry $registry
    )
    {
        $this->_registry = $registry;
    }

    public function execute(\Magento\Framework\Event\Observer $observer) 
    {
        $invoice = $this->getInvoiceObject();
        // My Customization
    }
    
    private function getInvoiceObject()
    {
        return $this->_registry->registry('current_invoice');
    }

} 

Check if it is helpful to you.
Magento version was 2.2.1

I have a similar requirement, I need to get invoice information after invoice save from admin.
I fulfil the requirement using the event observer.
app/code/Anshu/Customization/etc/adminhtml/events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="controller_action_postdispatch_sales_order_invoice_save">
        <observer name="observer_admin_invoice_save" instance="Anshu\Customization\Observer\AnshuInvoiceSave" />
    </event>
</config>

app/code/Anshu/Customization/Observer/AdminInvoiceSave.php

<?php

namespace Anshu\Customization\Observer;

use Magento\Framework\Event\ObserverInterface;

class AnshuInvoiceSave implements ObserverInterface 
{
    /**
    * @var \Magento\Framework\Registry
    */

   protected $_registry;
    
    public function __construct(
        \Magento\Framework\Registry $registry
    )
    {
        $this->_registry = $registry;
    }

    public function execute(\Magento\Framework\Event\Observer $observer) 
    {
        $invoice = $this->getInvoiceObject();
        // My Customization
    }
    
    private function getInvoiceObject()
    {
        return $this->_registry->registry('current_invoice');
    }

} 

Check if it is helpful to you.
Magento version was 2.2.1

I have a similar requirement, I need to get invoice information after invoice save from admin.
I fulfil the requirement using the event observer.

app/code/Anshu/Customization/etc/adminhtml/events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="controller_action_postdispatch_sales_order_invoice_save">
        <observer name="observer_admin_invoice_save" instance="Anshu\Customization\Observer\AnshuInvoiceSave" />
    </event>
</config>

app/code/Anshu/Customization/Observer/AdminInvoiceSave.php

<?php

namespace Anshu\Customization\Observer;

use Magento\Framework\Event\ObserverInterface;

class AnshuInvoiceSave implements ObserverInterface 
{
    /**
    * @var \Magento\Framework\Registry
    */

   protected $_registry;
    
    public function __construct(
        \Magento\Framework\Registry $registry
    )
    {
        $this->_registry = $registry;
    }

    public function execute(\Magento\Framework\Event\Observer $observer) 
    {
        $invoice = $this->getInvoiceObject();
        // My Customization
    }
    
    private function getInvoiceObject()
    {
        return $this->_registry->registry('current_invoice');
    }

} 

Check if it is helpful to you.
Magento version was 2.2.1