There are two ways you could use SMO in PowerShell 7 (cross-platform):
1. Installing the SMO NuGet packages, two packages are requiered:
a. Microsoft.SqlServer.SqlManagementObjects Version 150.18208.0 (as of 03/23/2020)
b. Microsoft.Data.SqlClient Version 1.1.1 (recommended)
2. Installing the PowerShell Module: SqlServer Version 21.1.18221 (as of 03/23/2020)
Keep in mind, once the packages and/or modules are installed, you need to update them manually.
Read the whole thing.