[junar]

To be honest, it depends on how much time you plan to spend on this.

If this is only dozens or low hundreds of person-hours, then just do things at your own pace. Don't worry about releasing a viable product. I'd guess that you find adding tax forms fun. But it's probably not fun to update every form each year, respond to bug reports, and handle other support requests, all under the time crunch of tax season.

(As a side note, if you want real impact with a time commitment in this magnitude, consider being a VITA volunteer! https://www.irs.gov/individuals/irs-tax-volunteers)

Of course, if you have thousands or ten thousands of hours instead, you should be able to support everything I listed, plus the features you'd find in other tax software. With that amount of time, you should be able to do enough market research yourself to figure out what to prioritize.