Solved: Leaving this up if anybody else has this same issue in the future. It was just me being stupid though, so not really likely to be of use for others to be fair.
I had not checked how the group field works and how to reference sub-fields in general, so I assumed I had to do some sub-array shenanigans. The solution turned out to just be the standard notation for referencing those values.
get_field('group_name_subfield_name') to get the field directly without having to get the field group first with an array of subfields
then to set the default value in a parent block using InnerBlocks it’s just that same key 'group_name_subfied_name' => {value}
