Suggestion for the Promote feature
I was unable to use the Promote feature for my project. I wanted to promote 300+ subsites.
All subsites were in one site collection. A feature in the UI to bulk select them is missing.
I had to change the url per site because they contained spaces, and the url is reused as Alias by ShareGate. It does not resolve the problem automatically. Also the UI does not support changing settings for all selected subsites.
I created a PowerShell script to automatically create a vallid url from the original url. But when using Promote from PowerShell in a for-loop, the Promote commandlet didn't finish even after a few hours, even though all content was migrated and available in the target site.
In the end I used PowerShell to provision empty Office 365 Groups and the Copy-Site commandlet to migrate from subsites to groups.