At least in Calif, as self employed I don't have to pay unemployment (think it is option) or worker's compensation (not eligible)
We all (self employed or regular) have to pay employee share of social security, something like 7% or 9%.
As self-employed, also the employer portion, which is the same. That's the only extra tax.
But first, expenses (work related vehicle mileage, my RF test and measurement lab) are deducted from gross income to get taxable net.
401(k) of all employers including self count toward the limit, also some federal plans.
Exception is 457(b); if you have that with your employer (state/county public sector) you can max that out, also your individual 401(k)
Your wife's SEP probably limits her contributions to a much lower figure than 401(k) would, because 401(k) allows 100% of income deferred up to the first $19k/$25k. Check it out, try running both in TurboTax or whatever, in time to decide this year if you want to open one. I think it could be a $15k difference in how much can be set aside pre-tax. At 50% marginal rate, that's $7500 more for you.