For those over age 59 1/2, you would need to withdraw all funds attributable to basis before your withdrawal would be sourced from Roth IRA earnings and the age of your Roth IRA would matter for taxation.
A SEP-IRA can be opened any time before you file your tax return and you can make prior-year contributions at that time. In contrast, a solo-401(k) must be opened and contributed to before the year end to count for that tax year.
In this podcast, I discuss my recent article “Account Funding Priorities for 2022” and describe a savings waterfall for 2022. The idea is when new money flows your way, which one of these buckets is it going to land in?