For clarity on timing on my answer: I’d try to time it so that you have enough time to get your taxes ready and the transcripts generate.
Assuming you get all 1099s on 2/15 and you or your tax accountant can do a quick turnaround and get everything filed before 3/15, that’d look like:
—12/30: upload everything to NVC
—1/31: DQ’d with the asset based form, sent to Madrid.
—Between 2/15-3/15: file taxes
—2 weeks after taxes accepted: transcripts generate
—Day transcripts generate upload new I-864 and new transcripts to CEAC
—3/31-4/30: interview in Madrid, bring updated I-864 and transcripts.
If needed, you could file 1040-X after the beneficiary moves but it might not be needed here if you originally file MFJ — standard dedication is above the I-864 threshold so if you’re at $31.5k or less there’d be no need to amend for tax reasons. Also for the record, nothing in this post is not legal or tax advice.