Thank you all for taking a look at my situation. I have done some further research but still got some confusing points.
1. Do IRS and states track the timing of stock sale? I did not make any profit in Q1 and Q2 so certainly did not pay estimated tax. If I have to pay estimated tax based on the calculation taking Q3 stock profit into consideration and I paid it in Q4 (after Oct 15 but before Jan 15, 21), will there be any penalty? If yes, would the penalty only accrue interest for 1 quarter (as there was no underpayment in Q1 and Q2).
2. Thank to this community now I understand IRS' rules determining estimated tax. Is it a simple method that I do not have to pay estimated tax if the other income (stock, interest) is less than 10% of my wage income with tax withheld? (Based on the rule that 2020 withholding > 90% of 2020 final tax). I know it won't be 100% accurate, but so long as the penalty is not material, I am ok.
3. For state tax, as the NY (where I work) state tax is withheld only on wage, am I right that I need to pay estimated tax to NJ (where I live) if the stock gain incurs over $400 tax to NJ (no need to consider withholding by NY)? As I found NJ determines estimated payment only by underpayment, not withholding.
4. I did not know why TurboTax asked me to make estimated payment to NY state. In 2019 my NY tax was quite accurate (only $40 underpayment). I checked the forms and found that TurboTax said that "110% of 2019 final tax" minus "2020 estimated final tax" is greater than $300. However, by the end of Sept, my NY tax withholding is still greater than my expected tax (based on wage income till Sept, standard deduction applied, no need to include stock gain). Can I just ignore the estimated payment? Doesn't TurboTax takes the % of withholding to final tax into consideration for NY state?
Thank you very much for your time, help, and patience!
