I hadn't thought about estimated payments! That might be a good backup plan if I can't get my employer to fix the withholding. Do you know if there's a minimum amount required for estimated payments? And what are the deadlines for those?

There's no minimum amount for estimated tax payments - you can pay whatever you need to make up the difference. The quarterly due dates are usually April 15, June 15, September 15, and January 15 of the following year (though they can shift slightly if those dates fall on weekends or holidays). Just be sure you're paying enough to meet one of the "safe harbor" provisions to avoid penalties: either 90% of this year's tax or 100% of last year's tax (110% if your AGI was over $150,000). I used the IRS Direct Pay system online which makes it pretty easy to submit payments, and you get immediate confirmation.

That works in theory, but some employers (like mine) seem to ignore even that. I explicitly put an additional $200 per paycheck on line 4(c) and they still somehow messed it up. It's like they're deliberately trying to cause tax problems!

Do you know if there's any catch to these free bank offers? Do they try to upsell you to premium versions halfway through or something?

Nope, no catch with the bank offers I've used. You get the full version of whatever tier they offer (usually Deluxe or Premier). You access it through your banking portal, and it's fully functional without upsells for the federal return. Some banks only cover the federal filing and you'll pay extra for state returns, so check the details. Also, these deals usually apply to online versions, not desktop software. But still a huge savings - just requires being a member of that financial institution.

Wow, that's kind of eye-opening and confirms what I suspected. I always wondered how much training the seasonal folks got. Do you think I'm better off with TurboTax then for a pretty basic return with just some basic investments?

If I choose actual expenses the first year, can I switch to standard mileage the next year if that works out better? Or am I locked into one method?

If you use the standard mileage rate the first year you use the car for business, you can switch between methods in subsequent years. However, if you use actual expenses the first year, you're locked into that method for the life of that vehicle for business purposes. That's why many tax professionals recommend using standard mileage the first year even if actual expenses might be slightly better - it preserves your flexibility to switch methods later if your situation changes. Once you choose actual expenses initially, you can't go back to standard mileage for that same vehicle.

Thanks for pointing this out! Do you know if driving to pick up prescriptions counts as medical mileage?

Do you know if establishing proper "economic substance" requires a physical presence in the Cayman Islands? Or would hiring local directors and maintaining an actual office there be sufficient?

Proper economic substance typically requires more than just a local address and hired directors. The Cayman Islands themselves have economic substance requirements that include things like adequate physical presence, locally-managed bank accounts, local employees, and appropriate local expenditure relative to the level of activity. The key issue in your specific case though is that if you're physically sitting in New York making the trading decisions and executing trades, it's going to be very difficult to argue that the economic activity isn't occurring in the US. The IRS looks at where the value-creating activity is actually happening, not just where the paperwork says it's happening.

You're right - there is no tax treaty between the US and Cayman Islands, which actually makes things more complicated. Without a treaty, there are fewer protections against double taxation and fewer clearly defined rules. Also, as a US-based trader (even temporarily), FATCA reporting kicks in at $50K for foreign accounts, and the OP mentioned potentially making $135K+ which would definitely trigger those thresholds.

I never even thought about that. Is it possible I missed out on refunds from my self-employment years too? I definitely had some business expenses that I'm guessing would have been deductible.

Yes, it's entirely possible you could have qualified for refunds during your self-employment years too. As a self-employed plumber, you likely had significant business expenses that would have been deductible - tools, supplies, vehicle expenses, mileage, possibly even a portion of your phone bill or home expenses if you used them for business purposes. The unfortunate reality is that if those potential refunds were from more than 3 years ago, they're likely forfeited. However, this is even more reason to get current with your tax filing - you might be leaving money on the table for more recent years, and you'll want to properly claim your legitimate business deductions going forward.

This is such an important point! I couldn't qualify for an apartment I wanted because I couldn't provide tax returns. Also had issues with my car loan application. Being off the tax grid causes so many unexpected problems.