How do I Bulk Edit Fields or Taxonomy of Nodes? I'm working on a site that needs bulk tagging on nodes. The D8 core View Bulk Operations Port is "simple" (https://www.drupal.org/node/1823572) and it doesn't have "modify entity values" which I used for this user-case in D7. There isn't a D8 tagged version of contrib VBO I can find.

My specific use-case is I need to bulk tag groups of blog posts with single terms. I have bulk imported around 2000 nodes from wordpress and need to apply new tags to them. Once the backlog id tagged I don't really need bulk editing any more. (But I'm sure many folks have similar use cases, like bulk editing fields or tags that aren't one-off tasks)

Actions is part of Core so it might be possible to write custom actions for my use case (Tag node with taxonomy term X, Y or Z). This documentation seems the most promising. https://www.drupal.org/node/2020549 

Can any confirm this is best way to bulk-edit nodes or is there a different approach that I should consider?
Drupal core doesn't provide this functionality, so yes, you need to implement it yourself.

Yes, you can implement your own action plugin to do this. You can also have a form to select which tag you want to apply. Have a look at the node delete action plugin, which uses a confirmation form, but you can implement whatever functionality you want in that form.

Keep in mind that Drupal core is missing another important feature for your use case. The ability to apply a bulk action to more than one page. The best you can do until there is a port of the contrib module to provide this to increase the amount of rows per page, so that you need to repeat it fewer times.