RDS Aurora Mysql Multi-master downtime and best practices

Hi all,

---Context--- A customer is currently using a large RDS Aurora cluster (12 replicas, 6x r5.12xlarge and 6x r5.4xlarge ) for their production environment. This cluster is currently part of a monolith that is proactively (and slowly) being broken down into smaller applications with independent data stores. This will still take months/years to complete due to competing priorities on their end

---Challenge--- Over the past few months the customer has performed a few database restarts due to either engine upgrades or different database parameter tuning. The customer would like to evaluate multi-master or any other alternative that mitigates service downtime as much as possible for future upgrades or restarts.

---Questions---

1. Is multi-master (2 nodes) + 12 additional read replicas an option at all?
2. If we ever implement a multi-master approach keeping the remaining replicas as readers, how does a database upgrade/restart affect the service? are all the database notes rebooted as well as it happens with a regular single-master cluster?
3. The customer application is not built for a multi-master active-active approach as they won't be able to handle deadlocks at the application level. Is a multi-master active-passive an option for fail-over?
4. Do we have any other recommendation/architecture for managing database upgrade/restarts that would help minimizing downtime?

Thanks!