Bulk/Cold Email Expert

We have a few brands under one umbrella and through competitions and channels we have over 40,000 emails. We would like to warm up these addresses for one brand in particular, however potentially a sister brand as well. Our core email tool is Klaviyo. However, as to not effect open rates and the IP for these accounts we thought SendGrid would be best and would like to use that to start with and them move engaged emails to Klaviyo once the leads warm up