How do I force a Writable Materialized View instead of an Updatable one?
Updatable materialized view was an Advanced Replication feature.
A writable materialized view was simply an updatable materialized view that was not a member of a materialized view (replication) group.
Desupport of Advanced Replication

Starting in Oracle Database 12c release 2 (12.2), the Advanced
Replication feature of Oracle Database is desupported.
The Oracle Database Advanced Replication feature is desupported in its
entirety. The desupport of this feature includes all functionality
associated with this feature: multimaster replication, updateable
materialized views, hierarchical materialized views, and deployment
templates. Read-only materialized views are still supported with basic
replication.

You are looking for a solution for a feature that became deprecated 7 years ago (in 12.1) and desupported 4 years ago (12.2).
The FOR UPDATE clause still exists for backward compatibility, but it is not even documented anymore.
I do not know why Oracle implemented it this way, but at this point, I would just let it go.
For the use case you described, a simple table is fine. You can do incremental refreshes with MERGE on key + timestamp, and do a complete refresh with truncate + insert ... select ....