What is the best practices to migrate SharePoint farm solution to SharePoint online?

For documents, we can either migrate manually or use a tool. Manually means to use one of the available methods for simply copying the files up, such as dragging/dropping files into a library, or using the sync tools. For a few libraries, this is fine. For larger migrations, most prefer to use a tool. MS just released a new tool, the aptly named SharePoint Migration Tool. This is new, so while it supports basic scenarios, you'll need to test it to see if it meets your needs. It should handle the basics, such as moving lists, libraries, and pages. Otherwise, there are several, robust, 3rd party tools on the market.

 

The CQWP should migrate fine as content of a page, as long as the fields it references and other supporting files it uses are there.

 

Managed Metadata is more problematic, as if terms are simply copied manually the IDs will be different between the environments. This will need either PowerShell or a 3rd party tool.

 

Workflows can also be tricky. If it's a small number of simple workflows, it's easiest to just recreate them in the cloud. For larger scenarios, the 3rd party tools are helpful.

 

As noted before, server-side web parts will not migrate. If you need the functionality, they will need to be rebuilt using available tools.

 

If by reports, you mean SQL Reporting Services, there is no Office 365 version of this tool. You'll need to either continue using SSRS on-prem, install it on an Azure VM, or move to a different tool for reporting, such as Power BI, and rebuild your reports.

 

All that to say it's still difficult to give specific advice. If it's one small site, just move things manually, and recreate the things that can't be moved. (And some people see the new functionality available in the new Modern sites and decide that they don't want to migrate as-is anyway, and choose to do a manual migration of selected content to these new sites). Otherwise, check into a migration tool, such as ShareGate, which will handle the more difficult aspects such as metadata and workflows, in addition to content.