You're not missing anything. The overall ratio idea doesn't really work. You have to report your own sales on your own tax return and your fiancée's sales on her tax return. Hopefully you have or can retrieve some kind of records of each sale. If you sold coins that were purchased with commingled funds, you would have to each report your proportional share as if it were two separate sales. You could use the ratio to calculate your shares. There might be no choice but to fudge your 2021 tax returns. You can probably get away with that this year because the crypto exchanges don't report transactions to the IRS, as far as I know. But there's a big push in Congress to require them to start reporting, so I wouldn't count on the lack of reporting continuing.
I will repeat, with stronger emphasis, my earlier suggestion to consult a local tax professional and see what they suggest. And get your accounting straightened out in the next week so you can start off right for 2022.
