WordPress was not designed with the idea in mind of multiple Themes on a single Site.
Which is why this plugin struggles to provide full multi-theme capabilities.
An alternative to this plugin is a WordPress Network, also known as Multisite. Each WordPress Site within a WordPress Network can have a different Theme. WordPress was built to fully support Multiple Themes used in this way.
What is less obvious, is that a WordPress Network of Sites can be designed to appear as if it is a single integrated web site.
For example, using the Subdirectory option of a WordPress Network:
Site 1 could be example.com, the web site’s home page, and any other web pages with the same Theme;
Site 2 could be example.com/forum, the discussion forum portion of your web site with a different Theme;
Site 3 could be example.com/news for a News section with its own Theme; and
Site 4 could be example.com/store with a fourth Theme for a Store.
Admittedly, extra effort will be required to make a WordPress Network look like a single web site, especially if you rely on automatically-created Menus.
Menu entries will have to be manually created to point to other Sites within the WordPress Network.