Custom attachment function not working in v5.4.2?

I have a function that I re-use in a number of custom themes when required to fulfil a customers need. I understand this can be achieved now with repeaters in ACF Pro but as I had a working function, I chose to use this.

The problem I’m now facing is I’m really stuck with getting this to show on a custom post type within v5.4.2. I even installed the ‘classic Editor’ plugin to see if it was an issue with legacy meta fields but no joy…

The code of the function with comments is:

I’m quite stuck with this so any tips would be much appreciated. I also need to serve the URL’s of these image files back via the v2 API. Does this requirement make it even more pertinent to use a plgin such as ACF Pro?

I worry about using too much 3rd party plugins when building bespoke themes not intended to be re-sold etc.