Help appending value to "Choice List" field - using python .append()

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How can I programatically append List Choices to a column if I cannot read the values in that field and write back to the same field?

For example, I have the following formula in my “$Tags” field, and it keeps giving me a circular reference error:

listTags = list($Tags)
if "covid" in LOWER($Certs_Disa):
  return list($Tags).append(["Covid"])