Hi @danielgray and welcome. Thank you for your positive comments 
You might be interested in this post (Which theme are you using and why? by @magetester) .
The only 2 themes that I have experience using in Magento 2 are:
- Porto (I use this for my main project)
- Ultimo (I used this during the Let’s Build Series)
I picked those based on their Popularity/Sales. Popular themes tend to have more pressure to get things fixed and supported quickly. Where as the less popular themes tend to have less features or get forgotten about by their creators.
Both have been an excellent choice with a lot of backend optimisations. They’re also well-built making it a little easier to make coding-customisations when necessary. Porto was less complicated than Ultimo to set up. If I were to build another store, I’d probably pick Porto over Ultimo.
But that’s all just my opinion.