There's not really a difference between side hustle income and career income. The IRS certainly draws no distinction.
You'll probably just be living off more of your side hustle pay and saving more of your work pay.
It sounds like you work for an employer (i.e. you have a 401k). If you could find a way to be self-employed, a SEP-IRA could be an option for you, which would change your maximum pre-tax savings to the lessor of 25% income or $57k. Depending on your specific numbers, that could be a bigger amount than the $19,500 you can put in a 401k. Of course there are a lot more considerations than that, including benefits, unemployment insurance, and social security.
Otherwise, just plow all remaining savings after maxing out your 401k and Roth into a taxable account. That's optimal.