Timing of capital Gaines taxes

when are those taxes due? Immediately upon sale of the mutual funds? Or on my yearly taxes next year?

the actual answer is that there will be no penalties for underpaying your taxes/owing come the tax return due date  if your withholding and timely estimated tax payments are either

90% of 2021 tax

or

100% of 2020 tax (110% if 2020 adjusted gross income is over $150K)

so if you meet either of the above you could end up owing $100,000 and there would be no penalty for owing this amount. it would have to be paid on or before 4/15/2022 (currently estimated due date for 2021 return) to avoid a penalty for late payment.

say 90% of 2021 tax is $180,000

but 100%/110% of 2020 tax is $100,000

and you timely pay in only $80,000

the underpayment penalty is only figured on $20,000 even though you owe $100,000 (based on 2020 penalty rates for this type of penalty it would be less than $400).

timely pay in $100,000 and penalty is zero though you owe $80,000

Whether you pay in sooner rather than later depends on how comfortable you feel knowing that you might owe a lot come 4/15/2022