For this: "Client wants to pay a wage to himself for 2021."

Has To Pay. It isn't a Want. Payroll is required for this person. If this entity was operating as an S Corp in 2021, yes, this was overlooked.

Yes, it's too late for 401(k). Or, are you considering Solo 401(k)?

https://directedira.com/2021_solo401k_contribution_deadlines/