Hi Tim,

First let me comment on this:

I'm trying to switch templates as I've done a few times before Gantry 5. Back then it was as simple as installing the new template in Extension Manager.

No, no... definitely no.
It has never been that simple. At least if I understand you correctly, what you want to achieve has absolutely never been that simply. Not with Gantry, not with Helix, not with T3, not with Warp, not with any other framework.

Changing a template in a Joomla website has never been "plug and play".

Now, here is what you need to do and what you have always had to do:

1. You need to install the new template via the Extension Manager.

2. You need to set the new template as "Default" in the Template Manager.

3. You need to configure everything as it is configured in the demo/quickstart. This step is something that you need to do with any template from any provider.

Everything is explained in details in the "Installation" section in the Construction documentation. I'm talking about the "Manual Installation" section there.
The next very important thing you need to do is to look at the "Home Outline" section in the Construction documentation and watch the video there (watch it carefully).
And finally, you should look at the "Modules and Positions" section in the Construction documentation.

Tim, it has always been like that - changing a Joomla template has always been a bit of a pain because you need to reconfigure some parts of your website (general settings, modules and module position, etc).