I know that the safe harbor for paying estimated quarterly federal taxes is 100% (or 110% if previous year's AGI > $150k) of the previous year's tax. So let's say you paid $40k in taxes in TY 2018, then to be sure you won't be subject to penalties for TY 2019, you would pay $10k (1/4 of $40k 2018 tax) on Apr 15, Jun 15, Sep 15 in 2019 and $10k on Jan 15 2020.
Now let's go forward to Apr 15, 2020. For tax year 2019 you have to file an extension on Apr 15, 2020 because you have investments in partnerships that are themselves on extension and you have no idea what kind of income they will produce. So what do you do then considering that you have no idea what your TY2019 tax will be? My best guess is continue to pay $10k in estimated taxes on Apr 15, Jun 15 2020. But then Sep 15, 2020 comes along and you finally get your K-1s and are able to calculate your taxable income and tax for TY 2019. Turns out you had a big year and your 2019 tax is $100k. So you have a balance due for TY 2019 ($100k total tax minus $40k estimated payments = $60k) so you pay the $60k balance on Sept 15, 2020. (As an aside, will there be interest due on the $60k balance for the time period from Apr 15 to Sep 15 2020?)
You've also underpaid estimated 2020 taxes (you've paid $20k total so far for TY 2020, but owe $100k for the year under safe harbor right?) So on Sep 15, 2020, do you then make an estimated TY 2020 quarterly payment of $55k ($25k x 3 quarters less $20k already paid), plus a final $25k on Jan 15, 2021? Will this still result in no penalties or interest due?
