Shopify Variant Dropdown Automation

I need help with my Shopify store using the Minimog theme. My art products have three variant types: Frame Style, Size and Frame Colour. Currently, I've hidden the unavailable variants, but I need the variant dropdowns to automatically select the first available variant and therefor always show available products and prices.

Here is an example product:
https://subtropicart.com/products/soft-hue-image

The Kalles theme can do this out of the box, I want this functionality for my Minimog theme. Please see screenshots attached of the variant switching in Kalles.

Key Requirements:
- I would like this to be a code fix, not an app installation.
- The store is print on demand so we don't need to play with the inventory.
- Unavailable variants are already hidden but if you are unhappy with the way it has been implemented you may re-write the code.
- There must always be a variant combination selected showing the price.

Ideal Skills and Experience:
- Experience with Shopify and the Minimog theme.
- Familiarity with product variants and dropdown functionalities.
- Strong problem-solving skills to ensure a smooth user experience.