Just posting this in case someone else has this same issue.
ISSUE: I had already added a bunch of records, then created a checklist with default items to check off for each, but since checklists only populate for new records and the system would not let me copy-paste to a checklist field or copy-paste all the items on separate lines, I could not figure out how to apply my checklist to hundreds of records in the system without adding each line separately (nooo - not a viable solution).
SOLUTION: Use automation! I created a new yes/no field called "Apply Action" (I will keep this in the table in case I need to do any bulk action again), then created a trigger of when the Apply Action field is YES to update the record with checklist tasks (you can add the checklist items in the action) then to update the Apply Action field to No. Then I copy pasted the yes to the entire column and the checklist was applied to all of the records.
If you only need to update some records not all, then only mark Apply Action "yes" the ones you wish to apply the action to and you can use your filters as well to filter out any records you don't want to update to get a clearer list to work with.
In future, I can use this Apply Action yes/no field to be the trigger for any action I want to do, so that seems helpful, too!