Hi @DAVIDPOWELL1 ,
Do you change the Choice field into Single Line Text type column in your SP List?
If you changed the Choice field into Single Line Text type column in your SP List, the original Choice field data card you enabled in your Edit form would be invalid.
Regarding the needs that you mentioned, I think you want to keep the field as Text in your SP List, but render it as a Dropdown box in your Edit form rather than default Text Input box, it's true?
I have made a test on my side, please try the following workaround:
1. Please remove the original added LargeCustomer DataCard from your Edit form.
2. Refresh your data source, re-enable the LargeCustomer field data card in your Edit form. Then Change the "Control Type" of this data card to "Allowed values" option, then the default Text input box would be converted into a Dropdown box.
3. Set the Items property of the generated Dropdown box to proper table values, e.g. ["Option1", "Option2", "Option3"], which would provide proper options to select from the Dropdown box.
Please check the following GIF screenshot for more details:

Please check above GIF screenshot, hope it could help solve your problem.
Regards,