"So it seems that I cannot use the existing unused basis from 2019 for 2025, and instead I need to do the calculations as if I correctly filed and claimed the basis each year, then use the new basis obtained this way for this year's taxes?"
Correct. A portion of basis was distributed each time even if it was not reported it correctly on Form 8606.
Each combination of decedent and beneficiary must be tracked separately. The issue with TurboTax trying to track basis in inherited IRAs is that there can be an unlimited number of such combinations even though it would be uncommon to have more than two. Tracking basis by IRA account doesn't work because of the aggregation rules. TurboTax has a mechanism for tracking only the basis of Taxpayer and Spouse, part of the IRA Information Worksheet. I'm not saying that tracking basis for each combination of decedent and beneficiary couldn't be done, but it would not be straightforward to implement.
