Apple - Open all iCloud tabs from iPhone on Mac

Assuming you don't require them to open literally simultaneously, but simply rapidly, this is quite doable.

First, add the iCloud Tabs icon () to your Safari toolbar, if it isn't there already. To do this, right-click the toolbar, select "Customize Toolbar…", and drag the icon to where you want it.

Once the icon is in your toolbar, click it. A list of tabs open on your other devices will appear. To quickly open lots or all of these pages in Safari, simply hold down the command key (⌘) while you click each site you want to open.

Command-clicking a link, bookmark, or other reference to a page will, by default, open it in a background tab. If you find, when you command-click one of the items in the list, that it opens in the foreground and closes the iCloud Tabs menu, either shift-command-click, or deselect "When a new tab or window opens, make it active" in Safari's tab preferences (Safari > Preferences… > Tabs).

Edit: The third-party tool CloudyTabs is a menu bar application which includes the ability to open all your iCloud tabs at once. (h/t Matt Sephton for finding it!)