Remove all the ISNULL with some other construct.

ISNULL is not SARGable and will cause RBAR which is what your seeing.

What is the client in Azure? What service?  Are they in the same region?  Are they in the same zone?  Are they in a proximity group?

You will need to check...

What do the logs in SQL and AVamar say?

One should give you a reason as to why the backup failed.

That’s not that difficult to create yourself and you would learn a lot more than someone providing you a script which may or not meet your requirements.

What are you struggling...

From the transaction log.

Any dirty pages lost in the buffer are redone via the log.

Check the event viewer and the setup bootstrap logs they will give information as to what is going one.

Although given how old that versions of SQL and the OS is,...

Well dates should be handled as dates not strings so the most important rule, store them in datetime columns not varchar/nvarchar columns.

SQL will then store the datetime value as a...

The account running the SQL service needs read/write permissions to your share as well as read/write permissions on the disk.

Set logins default language to British English instead of English.

Or as you should be doing, use ISO formats for dates and always provide them in a YYYY-MM-DDTHH:mm:SS.MS format

Don’t believe that is fully true anymore Jeff remember something similar but AWS now have procedures for backing up an RDS database to S3 so you may restore to another instance.

And how big is the biggest database, and how big is the biggest table?

Going off Jonathan's calculations @450 SQL should be @392 so not far off.

Do you have lock pages...

PAGEIOLATCH is reading data from disk to the buffer cache.

So what is flushing all the cache that everything needs to read back from disk.

How much RAM is in the SQL...

Backup the database(s)

move them to an S3 bucket

restore to your ec2 or Rds instance

ensure you script things like logins, jobs, alerts etc anything that’s server side.

you can look at dbatools...

Your missing something in the process then or something else in your setup isn’t correct.

https://www.sqlshack.com/restore-an-existing-availability-group-database-participating-in-sql-server-always-on-availability-groups/

1. Remove db from the group on primary

2.delete database from secondary

3.restore Vendor upgraded on primary

4.add database to group on primary with automatic seeding.

OR

1 remove db from ag

2 restore vendor database...