Sharepoint - SharePoint Online - Move Site and subsites into another site

My suggestion is that you utilize the site manager.

If you visit "http://yoursite/_layouts/sitemanager.aspx", you'll see a site manager, where you can do a move operation of all pages.

1) Select the parent site of the subsite you want to move in the left navigation pane.

2) Check the box next to the subsite you want to move in the right pane.

3) Click Actions drop-down and click Move.

4) Select Destination of the subsite selected in the next dialog.

We've purchased Sharegate to handle our site moves & migration needs because it wraps all the necessary Powershell scripts into a single GUI app. Since business requirements change periodically I found myself moving sites/lists/libraries more often than I'd prefer.

SharePoint Maven also has a couple options spelled out with pros and cons for each.