I believe contained database has the stored credentials and validation done on DB end. So you don't need system databases or users to be created.

MS brought in contained Db's to...

Mirroring is still available in 2019.

Log backup has to be done in sequence , if you want the log backup files for primary server recovery too, take the folder to...

Kindly post the screenshot. The windows should have the Login name/User found below on the error.

IF it is for a DR activity, why don't you user Log shipping or Mirroring?- Just for clarification.

The same code changes will apply here too.

We cant reset the default use accounts, these are individual to each server.

You cannot change post install also.

Post full backup restored as "No restore" option , it will accept only the next LSN differential backup, hope you are doing the same.

If in between some one restores a...

Though any of the logins in the cloud provider, else finding the server will be difficult.

Verify the logins for your team if any one of them/all are exposed.

Go though the link and try configure using performance conditions.

https://learn.microsoft.com/en-us/sql/ssms/agent/alerts?view=sql-server-ver16

What is the OS version and have you checked the below link for OS support.

https://learn.microsoft.com/en-us/troubleshoot/sql/general/use-sql-server-in-windows

I think you are expecting multiple availability groups yes possible for different Db's.

I recommend the same what Jeff suggested.

More over during recovery if you have only logs, you need all logs after full backup and in sequence to be restored.

but if you...

I dont know about AV working structure.

IS AV excluded to scan mdf, ndf, ldf , bak and trn files or respective backup files along with program file\ Microsoft SQL server...

check this link

https://dba.stackexchange.com/questions/36618/list-all-permissions-for-a-given-role

Readable secondary is set to read intent only which means allows on each node only when read intent is mentioned.

set each node parameter of readable secondary to yes and check.

IS availability group fluctuating?

if not is the permission level is same on both instances?

Kindly attach screenshot with AG configuration and error.