Hi Sir ,
I am Migration engineer and specialist in doing Tenant to Tenant Migration with various tool like BitTitan,Quest etc .
I would suggest to use BitTitan which can migrate mails, calendar, contact ,onedrive teams etc .It easy to setup and success rate is very high compare to other tool . I can drive this project with screen share and complete in 2 days .There is no downtime required . we need to plan final cutover for DNS once full migration will be done . I am happy to answer all your query over call if you want .
Thanks
Sumit