I opened up you Tracked Species and found 218 commas right away! You cannot have any commas anywhere, period. Got to get them out of there.
I also see / in your list. I would remove those too since if you ever use python on it, while it should be ok, it sometimes is not.
Whenever I have used external sheets you had to create a external_choices tab and use that. This may have all changed though as I started with v1.
External selects
The choice lists associated with cascading selects can become large and impact the performance of your forms. Survey123 has an option to save these lists in an external file and load them into the survey on demand.
To implement this option, create a tab on the spreadsheet for your survey named external_choices. This new tab should contain the same column headings as the choices tab. When the survey is published, values from the external_choices tab will be saved in a file called itemsets.csv in the media folder of your survey project, rather than as part of the form item. To reference the choice lists on the external_choices tab, use select_one_external to prefix the list name.
Note:
External selects are designed to work with choice lists associated with a cascading select only. Only choices with an applied choice filter will be added to the itemsets.csv file.