Recharacterizations of 401(k) contributions are not permitted. Going forward, though, you are correct that allocating some of your maximum elective deferral to be designated Roth contributions will reduce the deduction for the self-employed retirement contributions and will increase the amount that you are eligible to deduct for self-employed health insurance. Not only that, it can possibly increase the amount that you are eligible to contribute to an IRA since the amount of compensation available to support an IRA contribution is not reduced by designated Roth contributions to the 401(k).
And one additional note: SECURE 2.0 makes it possible for the employer contribution to the 401(k) now to be made to the designated Roth account as well. However, plans are not required to offer this option, so an update to your plan agreement would be needed to support this option.
