then it actually means I can contribute that much from March of 2025 until March of 2026
Not quite, it means your can contribute that much from Jan 1 2025 to March 2026. Anything you've declared on your 2024 taxes cannot be claimed on your 2025 taxes.
If in January you contribute 20k to your RRSP, you can claim 10k (assuming it's less or equal to your 2024 limit) on your 2024 taxes and claim the remaining 10k (your 10k contribution room for this year) on your 2025 taxes.
In other words, you don't have to wait until after March to contribute to your RRSP for the current year.