I'm not 100% clear on this but I think the pass-through income deduction applies regardless of whether it's a qualified business, as long as you're under the income limits ($157K single, $315K joint). The issue with qualified businesses only arises if you are above this threshold, where I expect few people on this forum will be. Is that correct??
When you combine this with the 20% "profit sharing" deferral that you get with a solo 401K on top of the annual 401K contribution limits, self-employment income becomes a real bonanza. The "hack" is to convert as much of your wages to self-employment income as possible. You'd have to compare this with job benefits though, as medical insurance is the obvious wild card.
Another note: if you're putting all your side hustle income into a solo 401K or SEP IRA, and accordingly reducing contributions to an employer 401K, you'll need to stop doing that. I have been doing this in order to shift savings to an account where I have complete control over investment choices and fewer hidden fees, and also to avoid having to deal with estimated tax payments.