"This relief does not apply to estimated tax payments that are due on April 15, 2021. These payments are still due on April 15."
For retirement, it is for the filing by the filing due date with extensions, so that is a relative deadline.
"Level Up" is a gaming function, not a real life function.
As far as I know the answer is no... I believe you can extend the due date of a Sep contribution for a c corporation by extending the due date of the c corporate tax return by filing an extension, i.e the extended due date of the c corporate return may be the date that the sep contribution is due.. But you should check this for yourself... Just my opinion