Upgrading postgres quickly, without downtime.

Hi admins,

I have a number of pg servers(~100 (each with a couple of replicas, so 300 clusters in total), each with ~2TB data) that is running version 10.

I am planning on upgrading to PG 15 and looking for ways to get it done without downtime and quickly.

There seem to be a couple of ways to do this -

1. dump/restore or upgrade -> but this causes downtime.

2. logical replication/pglogical -> this is going to be slow, adds additional load on the master, and it isnt recommended to do DDL during time.

I do have daily backups of these databases.

Is there a way I can use these daily backups to upgrade to 15 and sync the changes for the day alone?

I can code in C/scripts, and know bit of internals and wanted to ask where to start on this.

I am also open to any alternate ideas. 

thanks

karthik