"any tips on how to reduce client tax obligation"

Depends on how long of a prison term they would be willing to take a chance on. 

It's a little late to do any tax planning.  At this point it is kinda like asking the embalmer what can we do to save the dead guy.  All you can do is have them write the check and tell them to adjust their withholding for 2022 so there isn't a repeat next year.