Mageplaza Seo Ultimate package supports setting the default rating as 5 stars. Please configure as
Store > Configuration > SEO > Structured Data > Enable Default Aggregate Rating Value > Set Default Rating Value as 100

https://prnt.sc/qozp33

The default rating value has a scale of 100 so the closer you set it to 100, the more stars you get.

Updated on: 03 / 31 / 2020