#1 - if you can make it happen with payroll, do the $6400 to the 401K at work.
#2 - figure your business profit out - fill out a schedule C. You can open an individual 401K and use up the remainder of your annual contribution space and possibly some employer side as well there. You have to have the plan in place by 12/31 of this year, so get on that.
#3 - Finish up the Roth IRA as you've mentioned.
I'm thinking if a SEP is pre tax/a write off-contributing my entire self employment income to maxing a SEP for 2019 makes sense IF
This, is not actually an option. SEP is employer-side only money and is limited to 20% of your profit less 1/2 self employment tax (assuming you're taxed as a proprietor here - if you're an S-Corp, then it is 25% of your W-2 income). So there's no way to contribute the entire self employment income to a SEP IRA. You can, as mentioned above, do an individual 401K, and you might get close to that.