re: Solo 401k vs SEP IRA for new LLC - S election

I may be wrong. But for the SEP IRA, $58,000 or 25% of the salary can be placed in the SEP IRA transferred to a backdoor Roth IRA.

For a Solo 401k, $58,000 or 25% of the salary from the "employer" and $19,500 total from the "employee" so an overall greater amount but only the employee $19,000 can be converted to a Roth 401k.

SEP IRA: $58,000 max into Roth IRA
Solo 401k: $19,000 max into Roth 401k but $58,000 into the rest of the 401k

Wouldn't 58,000 pretax be better?