just to clarify when you make a backdated contribution to the prior tax year and convert to Roth, the Roth conversion part applies to the calendar year it is done, so if I'm following correctly from you've described you should have:
2024 - 5k contributions (in Q1 2025), no Roth conversion, 5k basis carryover
2025 - 7k contributions (in Q1 2026), 5k Roth conversion (maybe some earnings too?), 7k basis carryover
2026 - 7k Roth conversion (so far)
when preparing your 2025 taxes you wouldn't enter the 5k contributions, there should be a question whether you tracked basis and asks you for the basis from 2024 which will probably show $0 if you didn't have this in your 2024 filing & carryover - here you would enter 5k as the basis carried over from 2024 (confirm this lines up with your amended 2024 8606 Line 14), which will go to 2025 Form 8606 Line 2.
then enter your 2025 non-deductible contributions of 7k, 2025 Form 8606 Line 1+4 will show 7k and Line 3 will show 12k total etc.
you should have 1099-Rs for 5k (or more if you had some earnings between contribution and conversion?) so when you enter those and identify those at Roth conversions that will hit Line 8, Form 8606 will figure your tax and you should be left with 7k basis carryover to 2026.
in 2024 Turbotax I think you should be able to follow the amendment process (be sure to save copy of your original .tax2024 and PDF with all forms and worksheets from your initial filing - or help page below suggests doing a "Save As"), and enter the 5k non-deductible contributions and it should generate the 2024 8606 you can print and mail, but you shouldn't need to file amendment if tax was not affected - see notes at end of the help for Backdoor Roth page below for case when you didn't file prior year 8606.
https://ttlc.intuit.com/turbotax-support/en-us/help-article/retirement-benefits/enter-backdoor-roth-...
https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-a...
hope that helps get pointed in the right direction.
