Solo 401k / SEP IRA contributions

Trying to help a family member:

Full-time job W2 maxed employee contribution of $19,500, employer matches a small amount

Second part-time job, have a singe member LLC, non-W2 income.

Interested in maximizing tax deduction and catching up on retirement savings.

Did a SEP IRA last year. I think the SEP IRA was something like income-expenses and then could contribute up to 20% of that. Is Solo 401k better? Can another $19,500 employee contribution be made? Or since that was maxed out as W2 employee the only option is to make an employer contribution under the LLC up to $58,000? What is the max that can be contributed via Solo 401k? More than SEP IRA?