Hi @blondIT - In this situation best option according to me is to have Dimension tables in Dual mode, Fact table in Direct query mode. Add Aggregation tables in Import mode to improve the performance. For more information on Aggregation table read the below Microsoft article.
Follow Power bi best practices like keep only required columns, use Incremental refresh etc.
Also, study on Direct Query performance improvement recommendations. They are lot of articles available on this.
Thanks
Ankit J