The §645 election (Form 8855) is due by the due date (including extensions) of the first Form 1041 for the related estate/fiscal year—not by the decedent’s 1040 due date. For a fiscal year ending 9/30/2025, the first 1041 is due 1/15/2026 (15th day of the 4th month after FYE), so 8855 is due the same day (or later if you extend the 1041)

It is not necessary (and doesn’t help “matching”) to file the 1040 and 1041 on the same calendar day. 1099 matching is TIN-based, and your 1099-B is under the EIN. File the 1040 by 10/15/2025 as planned. Then mail/file Form 8855 and file the 1041 any time on or before 1/15/2026 (or on extension). This is the cleaner path and ensures the §645 election is properly on file before (or contemporaneous with) the first 1041.

If an executor is appointed after a trustee makes the election, be aware a revised 8855 may be required within 90 days if the executor agrees; otherwise, the election can terminate.