Is EXPDP the best backup approach when running Oracle on Windows with no RMAN?
When you install an Oracle database RMAN is installed along with it. This has been the case for many years.
As I work at the Enterprise level backup has a fairly restrictive meaning and an RMAN backup is the only backup solution that Oracle supports.  Even third party solutions like Commvault still use RMAN commands to backup the database.
Dump files are very useful for refreshing development but you cannot do a complete restore of a database from a dump file.  You would need to install a new database first. See this excellent answer from Phil here
Ultimately the answer is what is required for backups?

Is there a mean time to recovery?
A disaster recovery plan?
SLA level agreements to clients?

If your shop has to answer these kind of questions then best practice is RMAN backups copied to another server with older backups sent off site and aged out after so many days.