Migration time could be a few minutes for that amount of data. I would be surprise if it takes 15-20 minutes. I have migrated 750 GB in about 50 minutes.
The Report Server database can be hosted on Azure SQL Managed instance without any issues while the SSRS is installed on a VM. See SSRS documentation.
About SSIS, SSIS to Azure Data Factory (ADF) or a VM (the SSISDB catalog can be hosted in the managed instance). To “lift and shift” your SSIS-based ETL you need to configure Azure Data Factory (ADF) to host the SSIS runtime. Take in consideration that only SQL Server Management Studio 17.7 and above have support for SSIS package scheduling in SQL Agent on SQL Managed instance. With that version of SSMS or above you can create SQL Agent jobs to execute SSIS packages on the managed instance.
About pricing or discount price for testing, Dev/test pricing is available for SQL Database Managed Instance as part of the Azure Enterprise dev/test offer, providing a cost-effective way to run your development and testing workloads on Azure SQL Database. With dev/test pricing for Azure SQL Database, you can save up to 55 percent versus list prices.