Unfortunately, I think it is relatively common for people who can work only by tweaking existing templates and frameworks to call themselves a "developer." Your post reminds me of this article:
devwp.eu/development/dont-call-yourself-a-develop…
The author discusses the exact problem you bring up in your question. I tend to agree with him, and I'm also sure that Jason Knight can say a lot about what he thinks of the article. ;)
In any case, I think the people you hired cut a lot of corners and overcharged you for their work. It's a red flag when I hear stuff like someone using an off-the-shelf premium theme for a WordPress project that's supposed to be a "full-custom" job. I wouldn't pay any more than $30 for someone to customize it for me if I were in your situation and that's what you want. (= Hire someone to customize for you so you won't need to mess with it yourself.) But then, I wouldn't even use such shortcut myself. WordPress premium themes are bad news in my past experiences using them.