To avoid underpayment penalty you need to pay tax during the year thru withholding or timely (usually, quarterly) estimated tax, the smaller of either 100% of your 2024 tax (110% if AGI > 150k), or 90% of your 2025 tax - this is your 'safe harbor' amount.
If you are using the ES voucher amounts generated by TT 2024 when you filed your 2024 return and didn't provide estimates for 2025 (Other Tax Situations / Form W4 and Estimated Tax), by default these calculations are based on the 100/110% of 2024 tax, and assuming withholding for 2025 is the same as 2024. Suggest confirming the amount you are paying for ES and which method/safe harbor it is based upon and keep in mind this calculation in TT was based on a certain amount of withholding so check that is correct for 2025 also.
So for question #1 I think the answer is No you don't need to increase ES if you are paying based on "100% of 2024". For your situation it's ideal for the "100% of 2024" option as the lower 'safe harbor', as long as you pay the ES necessary to meet that on quarterly basis you do not need to pay the additional tax due for 2025 until you file your return, regardless the amount of additional income or when it occurs.
Question #2/3 - I don't think these apply if you're paying evenly based on 2024; but in case it comes up again, in general if you underpay or miss Q1 the minimum you need to pay is 50% worth of the total ES by Q2 which will not erase the Q1 penalty but will stop the penalty from Q1 accruing interest beyond Q2, and then resume the quarterly 25% for each of Q3 and Q4. If you miss Q1 say and then split your total ES into thirds you will still be underpaid in Q2 as you need to pay 50% by Q2 not 33%, and underpaid in Q3 as you need to pay 75% by Q3 not 66%. IRS is assuming by default your income occurs evenly each quarter, also assumes withholding was paid evenly thru the year, and ES need to line up with that by quarter. The penalty is calculated for each quarter, underpayment is not eliminated by overpayment later (but you can stop the penalty accruing as soon as you address the underpayment situation); but overpayment in earlier quarters will carry forward and offset underpayment in later quarters.
If you were paying the ES based on an original estimation of 90% of your 2025 tax before the cap gain, you can switch to the 100% of 2024 tax ES amounts, and pay in Q2 whatever gets you to 50% of that including your payment in Q1. You may still have a small underpayment penalty for Q1 in this situation but will only be for 25% of the ES for a few months.
The other alternative if you have a large income event and are paying ES based on 90% of 2025, is to file using the Annualized Income method on Form 2210 (via Other Tax Situations/Estimated Taxes) but can involve some work as you have to provide AGI, withholding etc by quarter (by 3/31, 5/31, 8/31, 12/31). This method also then requires that you pay all the tax in the same quarter as the associated income so everything lines up. It's useful method if you have a large income event late in Q4 say (e.g. Roth conversion, cap gains) but couldn't plan for this and then need to pay one-off ES in Q4. In your case since it occurred earlier in the year and if you are paying using the 2024 tax which is fixed and known amount, you shouldn't need this method, and should be able to avoid paying 60K to the IRS now that isn't due til April '26.
Question #4 can't speak to GA tax but check the state requirements for Estimated Taxes, hopefully they have the same provision to pay based on 2024 tax.
Your question says home & biz version so I think this next bit will apply, but if you are using desktop TT you can use the 2024 version to do a new dummy return as an estimate for 2025 and check all this underpayment penalty math, you just have to allow for different deduction limits and tax rates that would apply in 2025 but you will be able to get close. You can't carryover a return so you would have to input your 2024 info like the AGI/tax for 2024 as tho carried over from 2023. Under Other Tax Situations / Underpayment Penalty you can plug in the prior year AGI/tax and it will kick in the lower safe harbor and eliminate the penalty. In Forms mode if you double-click on Line 38 in Form 1040 and double click again on the worksheet, it will bring up Form 2210 calculations and you should see the safe harbor and penalty calculations there.
For more info on estimated tax see https://www.irs.gov/faqs/estimated-tax
For more info on Form 2210 penalty calcs see https://www.irs.gov/pub/irs-pdf/i2210.pdf
Not a CPA just my 2 cents on this, pls research accordingly, hope this helps.
