You can update your checklists via Bulk Issues Update with the help of Automation for Jira tool.
This goes about Issues bulk update when you set a value for a field you'd like to populate to all the selected issues.
The filed you’ll be populating to update checklists is “Checklists“. Check the field setup instructions here.
Do it before running a bulk import
Go to Automation for Jira → Automation Rules
Click Create Rule
Set When: Field value changed Change
Choose Checklists → Save
Add component New Action
Choose Set entity property
Choose Entity type: Issue
Set Property Key
Add the following to the Property value
Save the rule and Turn Automation On
Proceed with the bulk update.
Once the bulk update is completed, turn the rule off.
Make sure this rule is only enabled when you need it for the bulk update operation. Otherwise, it will constantly run the synchronization, and you don't need that as a normal checklist operation would suffer.