Hi @Suggy ,

incidents and problems ( whichever is not workflow driven can be brought into servicenow)
I doubt there would many change requests among those 1k active task records( users can work on the old tool to and close them) or they can raise a new one in the servicenow ( considering the no.of changes)

of requests you can use (service catalog api or cartJS) 

this is what we have done when migrating from an onprem JIRA to ServiceNow

Please mark my answer as helpful/correct if it resolves your query.

Regards,
Chaitanya