Migration Tools (specifically ShareGate and AvePoint)

I've only ever used ShareGate for this type of migration, and while it is faster than manually moving everything over one by one, you'll need to be aware that any little change in environments can often result in you having to go through each Form / Workflow anyways, in order to make sure that it's committed and working. 

In my case I believe I had made a duplicate of the site first (manually, but with all of the same columns / setup), and tried to then port over some Nintex stuffs from a Test server to a Production Server only to have it think that certain columns were not there / missing. 

I later found that it would have been a better idea to just port over the entire Site / List / Library whole-cloth from the Test to the Production using ShareGate, but you live and learn :) 

Beyond that though, I have found sharegate to be an extremely useful tool in just managing several sharepoints at once. Especially if you have multiple versions and information that is stored in one place, but not another. It makes moving data between servers an absolute BREEZE, and using the mapping tools you can do nice stuff like automatically fill in data that would otherwise be missing (like if a person is no longer with the company and therefore no longer in AD. You can default instances where a person isn't found with a new user). 

Hi,

I used Sharegate to migrate close to hundred site collections with many number of sub sites and Nintex Workflows, Forms; 

Sharegate did great work in migrating sites, providing migration logs, friendly and quick response customer support, and more importantly, PowerShell support, documentation with syntax and examples.

1. I have automated all of my migrations using PnP-PowerShell (to create sites first on SharePoint online with required templates) and then Sharegate PowerShell (to copy structure).
2. Sharegate migrated all of my Nintex Workflows (except non-supported actions, UDAs)
3. Sharegate also migrated Nintex Forms (except responsive forms - List items with responsive Nintex forms can view/edit/create with little/no disturbed design, but the responsive form can not be open in Design mode, it keep spins to open the form, but never opens - checked with Sharegate and Nintex, they are not supporting at that point of time - may be few months ago, not sure if they support now).

Ditto