@superlyc #2 - your proof is the email confirmation that the IRS sends you and the debit in your bank account statement and it would be listed on your IRS transcript.
#3 - if you are going to owe an underpayment penalty, then pay now as it will cut off the accumulation of interest charges. Otherwise, pay by January 15, 2026.
#4 - a CPA is unlikely to be cheaper. Just pay your estimate now and then assess what the underpayment penalty looks like. It is possible there would be no penalty due to the 'safe harbor' option.
