Combine two lists into one before "to_json" modifier

There might be another way of solving this than combining 2 list into one but right now I think I need to merge it so I only get one group of []

I am using a Grid field so I have have a heading above my lists. For example a recipe instruction where I want add "Filling" before a list of instructions and then "Pastry" before another list of instructions.

Here is my fieldset YAML.

  instructions:
    type: grid
    display: Instructions
    fields:
      part:
        type: text
        display: Part
      instructionlist:
        type: list
        display: Instructions

This is great as I can do this frontend for visitors:

Filling:
- blend
- cook
Pastry:
- mix
- bake

Great UI for the visitor (at least I think so) but now I need to build the JSON LD Microdata for Google and I dont want to let Google dictate that I cant add a heading (Filling & Pastry for example) and thereby splitting up my list but I am running into a problem as when using to_json modifier I am getting two (what the correct term?) groups of [""] as seen here:

["blend","cook"] ["mix","bake"] 

Where I would like to only have

["blend","cook","mix","bake"] 

Here is my frontend code

{{ instructions }}
   {{ part }}
   {{ instructionlist | ul }}
{{ /instructions }}

Here is what my to_json code

{{ instructions }}
   {{ instructionlist | to_json }}
{{ /instructions }}

And here is the content from the MD file

instructions:
  - 
    part: Filling
    instructionlist:
      - blend
      - cook
  - 
    part: Shortcrust Pastry
    instructionlist:
      - mix
      - bake

I might be approaching is all wrong but after spending a long time looking at fieldset ideas and modifiers I have given up.

In case you are interesting in the CP view of the fieldset then here is a screenshot

Thank you in advance