Hello @Ben_Sefton and welcome here!
Thank you for your introduction.
I look forward to reading your future articles. It seems we share some use cases (water infrastructure monitoring and pasture assessment).
If I understand your requirement correctly, you want to filter the options of a select_multiple question based on a previous answer ( -> choice_filter) and then select all those options by default?
Here is something that seems to work fine :
test_default_select_multiple.xlsx (475,8 Ko)
You can also take a look at this showcase : Pre-populating select with default option