The TurboTax developers have not yet fully updated the Keogh, SEP and SIMPLE Contribution Worksheet to reflect the 2020 limits. Expect to see that updated in a future release. This is a typical occurrence every year; unfortunately, you'll just have to wait for that update.
Income reported on a W-2 is not income from self-employment and does not contribute to the compensation from self-employment that is used to calculate your maximum contribution to the solo 401(k) that is reportable as a self-employed retirement deduction on your individual tax return. I don't think that you were indicating that you included that, but I just wanted that to be clear.
All of your income and expenses from all of your self-employment work, including the $2k from your side-hustles assuming that that is not the income reported on your W-2, must be reported on Schedule C. It is all subject to self-employment taxes.
