Hi Charlie,
I had a look at the site and I was able to solve the minor Wappler errors you got but there is a big issue with such large custom templates.
First it is a Bootstrap 3 very custom build template. It contains hundreds of bloated jquery components and tweaked styles with very custom CSS. It is also build with a large pack of custom layouts called 300 chocka blocks specifically for Pinegrow.
You will never be able to convert such template site to Bootstrap 4 because it is so custom made.
Also the site is full of 99% bloated components and layouts that you actually never use. This makes the site terrible heavy and difficult to use and maintain.
If you can maintain such bloated templates in Pinegrow - you should really keep it in there, specially because you are using components specially made for Pinegrow.
Actually your site is very basic in structure and content, so I wonder why you needed all those bloated components and way over complicated layouts in first place.
You can make very similar website much faster from scratch in Wappler and it will be clean, easy to maintain and really have only the layouts of functionality you need. This will make your site faster and much more easier to use.
So I suggest that instead of trying to convert old template based sites, you start from fresh in Wappler and learn how to create modern, responsive and fast web sites. This will pay off in the long term.
If you don’t have the time to recreate the sites - just leave them as is and focus on creating new ones with Wappler.