goursh

Easy!
 
Thank you @Redro and @Rocio0010, I didn't realize you receive an extension letter right away. (I suppose it was silly of me to imagine that an interview appointment would be given within the next 3 months!)
 
Consider this topic solved

1. Pack your bags, make sure you lock the house.
2. If you have any pets, do take them with you at least within the States.
 
None of the above.
 
Just submit the application when the time comes, with the address in State 2. Send it with proof of delivery. Travel with your extension letter. Then, continue to live your life because you won’t 

When people file I-751 they receive a 48 month extension letter. 
You do not have to inform anyone of your planned travels because you will more than likely be maintaining ties to State 2 (and keeping your home in State 2).
And will be back in US state 2 before anything meaningful happens. 
If things stay the same with I751 processing times, your wife will only receive notice of an interview late 2024 or early 2025. 
You only have to inform USCIS if you move permanently. 
Make sure your wife has her extension letter when she travels back to her home country. I assume her GC will expire while she is out of the country. 
@Rocio0010 is a pro at traveling with extension letter.

Thanks everyone for your feedback. I've slept on it after reading your very good points, and it's probably best just to see how things pan out in the next 2-3 months before deciding on a dramatic change of direction. Cheers!

Fear sells

No such thing as declaration of how we met.  Circumstances of meeting can be but a couple of sentences and is not signed 
 
USCIS is NOT interested in an emotional mumbo jumbo love letter