Actual CPA here, you'll owe an underpayment of estimated tax penalty if you don't pay in at least the lesser of 90% of current year/100% of prior year (110% if AGI >$150K) tax. The penalty is based on the Federal short term rate and prorated based on amount underpaid and time outstanding (i.e. Q1 is overdue for the whole year but only 1/4 of the underpayment, Q4 is the whole amount but only late by one quarter). When all is said and done, it usually works out to like 2% and we have plenty of clients that would rather hold the cash and pay the penalty because it's generally cheaper than borrowing, or they figure they can get a greater rate of return by interesting it (depending on their situation/perspective).

If you miss April 15th it goes to credit card rates though (failure to pay penalty, which is very different from underpayment of estimated tax). Don't do that. Remember: an extension is for time to file, not time to pay.

I feel like there needs to be instance aggregation for Lemmy to really work in the long run (and really this is probably true of the fediverse in general). Having to add communities across multiple instances, and not being able to browse them in a centralized way, really detracts from the experience. On Reddit, I subbed to the stuff I wanted and just lived off that feed. With Lemmy, I feel like I have to stay in unfiltered view to get anything of interest--the fragmented niche communities are just too limiting.