Understanding Storage Account replication downtime

I have a Storage account that's used as a CDN to host a  lot of generally small files which occupy about 2GB. This is a small but critical part of our application which is used heavily by our app but which has no redundancy (it currently only has LRS replication).

It's hosted in UK South and while Storage Accounts are very reliable, I'm concerned that if there's ever a regional outage there's nothing I'd be able to do.

The requirements therefore are:

• Convert it from LRS to GZRS i.e. actively replicating from UK South to UK West.
• No app changes required to detect when the primary goes down and to switch to the secondary-this needs to be transparent.
• No or low downtime when the change is made.
• We need to be able to write to the secondary after failover. As a software company anything that limits our ability to push code changes is not acceptable, so RA-GZRS is off the table.

After doing a bit of reading, I found the following warning in the docs:

If you choose to perform a manual migration, downtime is required but you have more control over the timing of the migration process.

https://learn.microsoft.com/en-us/azure/storage/common/redundancy-migration?tabs=portal#downtime-requirements

This is typically light on detail and leaves some critical questions unanswered:

1. Is there any way of estimating how long the downtime will be so I can appropriately set expectations of management and customers when scheduling the maintenance window needed?
2. It specifically mentions manual migrations i.e. making the change through the Azure Portal, would making the change through IAC e.g. Bicep or Terraform be any different?

Any input from anyone who's made any similar changes will also be appreciated.

Edit: I've just checked and found that UK West still doesn't have Availability Zone support, is my best option for reducing the risk of this single point of failure to set the replication to GRS?
https://learn.microsoft.com/en-us/azure/reliability/regions-list#azure-regions-list-1