one item you mentioned.... the brokerage firm selling stocks that your wife inherited..... for any stocks purchased prior to the date of death, the cost basis "steps up" to the price on the date of death. Also, they are all considered long term gains / losses even if they had been held for less than a year. So you are only paying capital gains tax measured on gains from the date of death forward.....
further, if you think there is a potential for interest for late payment, suggest making the estimated payment now and not wait until January 15th. The IRS measures this quarterly and if there are interest charges for late payment from the Sept 15, 2020 payment that should have been made, you can cut off that interest clock by paying today.
