Hi Maity,
Using of SLT tools will definitely help to reduce the downtime but it comes with additional cost of license as well as implementation.
You need to check how much business downtime is permissible in your environment with addition of CPU and RAM , with various mocks try to achieve the desired results.
One suggestion is to combine upgrade as well as platform migration back to back to help reduce downtime.
Hope this helps.
Regards,
Deepak Kori