[Upgrade to 4.3x] Advice on Upgrading

I am planning to upgrade to 4.3 from 4.1x.

In the past, I made a copy of my existing database and a copy of the files. I import them into a test environment. I customize the templates, applications, plugins and configure the settings. When all is working and I'm satisfied, I upgrade my live forum then install or upgrade the applications and plugins and configure the settings.

I want to upgrade differently this time around by installing the new theme, customize the theme, install the applications and plugins. Configure the settings.

Then after all is finished, I'd like to import all of the data.

What is the best way to go about this?

If you have other recommendations I'm open ears.

Please advise.