Hi, please think twice before using Boundless. Some of my friends used it and got their immigration benefit without and issue. However, AFAIK they're not certified immigration attorneys. They're a big factory for filling the forms. You may be better off filling forms yourself to avoid mistakes. You may want to look into VJ partners or immigration lawyer on official AILA website. However, you do not need a lawyer for sure, many VJ users DIY'd the process.
You need to establish US domicile, but that won't be too important until the next part of process, which is getting immigration visa after I-130 approval. You can continue living together for most of the time I-130 is pending. But make sure to go to the States, get the lease, Driver License, register to vote, find job closer to I-130 decision.
Also, regardless of joint sponsor, you will always remain the main sponsor and will have to provide copies of tax return transcripts. Have you filed US taxes all this time living overseas?