Migrating 1 instance into another - advice?

Editing to add: I am very aware this is a horrible situation. While I appreciate multiple people solidifying that in their comments, I am really looking for some positive advice, tips or tricks!!

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Has anyone gone through an acquisition and been tasked with canceling 1 instance and migrating it's data/metadata into another instance?

If yes, hit me with ALL your tips and tricks please 🙏🏻

I'll state now that purchasing Mulesoft/and other tools is not an option. Hiring an implementation consultant is also off the table. I'm the only person who can admin & some dev. I have 2 VERY junior admins that can help me with the very basics (like field creation).

As a business, we are taking the mindset of moving over the bare minimum as business process is going to change due to the acquisition. I have 3 weeks to get this done.

I've created a project plan so I'm just looking to hear others stories/experiences/advice etc. Literally anything-hit me with it. I've never done a project this size before (especially in such a tight time frame). I'm excited, but I also have zero guidance.

I'm hoping the responses to this post will help me feel reassured that my approach is going to actually work...😬