Underpayment tax penalty because of long term capital gains-

try working through Form 2210AI....

the IRS assumes equal income throujghout the year and determines the penalty on that basis

however form 2210AI permits you to reflect 'lumpy' income.  In your case it's 'lumpy' because you have this one time event in July.  That should at least reduce any penalty as there was relatively little income in the 1st half of the year

Penalties are also based on your prior year's tax liability. The general rule is that you need to pay in 90% of the current tax or 100% of the prior year tax. So if you owe $15,000 this year and only paid in $10,000 you would be subject to the penalty because you did not pay in 90%, but if last year's tax was only $10,000 there will be no penalty due. So maybe find where to put in last year's tax due.

Just a question/something to consider.

Who calculated the gains to be reported by the remaindermen?   It has been a long time since I've looked in that area, but as I recall, that is a somewhat complex calculation to split the gain between person who has the life estate and the remaindermen.

You might want to review those calculations and how they were done to make sure it was done correctly.