is this in the retirement section for Form 8606 - this form is very specific about dates and handling the Q1 contributions, so take the instructions very literally when it asks for your balance as of 12/31/24 it means as of that date. Form 5498 is usually not mailed until May but the instructions for Form 8606 refers to using statements provided in January i.e. your brokerage statements for 12/31/24. Form 5498 should also be the balance as of 12/31 and should be same as your statement. It just happens you're filing with extension so you do have the 5498.
Roth conversions are taxed by the calendar year in which they are done. If you made a non-deductible contribution to your Trad IRA in April 2025 for 2024, that counts towards your 2024 limits and becomes part of your final basis on Form 8606 for 2024, carried over to 2025. The Roth conversion you then immediately did of that contribution in April 2025 applies to the 2025 tax year not 2024 (along with your 2025 backdoor if you do it during the calendar year) - you won't have received the 1099-R for it yet.
https://ttlc.intuit.com/turbotax-support/en-us/help-article/retirement-benefits/enter-backdoor-roth-...
https://www.irs.gov/pub/irs-pdf/i8606.pdf
Line 6
Enter the total value of all your traditional, traditional SEP,
and traditional SIMPLE IRAs as of December 31, 2024,
plus any outstanding rollovers. A statement should be sent
to you by January 31, 2025
