"can I then, in addition to that 401k contribution, make a contribution to a Roth 401k in the amount of the health insurance deduction?"
Yes, subject to the overall limit on elective deferrals and employee Roth contributions combined. The amount that you are eligible to deduct for the self-employed health insurance deduction is not reduced by the amount of an employee Roth contribution to the 401(k).
"Couldn't I set up a Roth 401k before then and make a 2023 contribution into that account?"
The "Roth 401(k)" is not a separate plan, it's a separate designated account within the 401(k) plan, plan permitting. If you already have a Fidelity self-employed 401(k) plan established for 2023, that plan does not support having a designated Roth account. If you have yet to establish a 401(k) plan to receive your contributions for 2023, you could establish one with a provider that has a plan that supports a designated Roth account. For a first-year plan, you have until the due date of your 2023 tax return (without extensions), to establish a plan to which you can make an elective deferral or employee Roth contribution by that deadline.
