In your case you have been told that you do not receive any interest until maturity so you are absolutely correct in waiting to include it all in your 2025 tax return.
However, if you want to include the portion that was earned in 2024 on your 2024 tax return then you are fine to do so. You should be able to figure out what portion of the total interest that will be earned at maturity has been earned by the end of 2024. Then, when it matures in 2025 and issues you a statement for the full amount of interest earned, you will just deduct the amount of interest that you reported in 2024.
Keep in mind that if you are paying taxes on this interest in Spain it may be best to declare the income at the same time that it is reported there. The US foreign tax credit gives credit for the lesser of taxes paid or the equivalent US taxes on the foreign income. By splitting up the interest on two separate years you will reduce your foreign taxable income in 2025 and may not be able to take advantage of the full foreign tax credit to which you would otherwise be entitled.
@Juancar
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