@mcelwest , yes, everything you said is correct except that the regular deferral limit for 2020 was $19,500, not $19,000. Had you set up a solo 401(k) before year end, you could have made elective deferrals from the net earnings from self-employment up to an amount that would bring you to a total of $19,500 of elective deferrals between your former employer's plan and your solo 401(k) plan combined, assuming sufficient net earnings. With sufficient net earnings beyond the amount of elective deferrals, you could have made the same employer contribution of 20% of net earnings to the solo 401(k) that you can make to a SEP IRA for 2020.
Note that you could establish a solo 401(k) now instead of a SEP IRA and make the employer contribution for 2020 to the solo 401(k), so no need to establish a SEP plan for 2020 and then switch to a solo 401(k) for 2021. You just can't make an elective deferral for 2020 because the solo 401(k) was not established by year end. (The election to make the deferral is required to have been made by year end and you can't have made that election to a plan that did not yet exist.)
