I believe the annual limit for a 401k applies to all your accounts, so if you are already maxing out your 401k at your main job, you couldn't contribute to a solo 401k. But you would still be eligible to contribute to a SEP.
I have a SEP for my side gig, and my wife has one as well. I also max out a 401k at my main job and Roth IRA's for us both. A SEP-IRA is super easy to set up and maintain, almost no paperwork.
The first year we set up my wife's SEP, we could only contribute $1600, but that lowered our federal tax by $800. Effectively an $800 contribution with a 100% matching gift from Uncle Sam.
The most you can put into a SEP works out to be a little less than 20% of your self-employed income, which is okay for us. If we had more money to invest, I would be looking at a solo 401k for my wife.