I have been here many times. Unless you use a noncompete that works (see the proposed Section 457(f) regs for the narrow circumstances where this may work), once the retired founder has a legally binding right to the payments (which may be when they are put it in writing, or, depending on the facts and circumstances, may even be now or some time in the past), she will be or was taxable immediately on the present value of the payments. The future payments will also be taxable, but offset to a great extent by her basis resulting from the large up-front tax payment (only the deemed interest resulting from the discount taken to determine present value will be newly taxable). The agreement will also need to comply with 409A (mostly, require fixed payments, which is what is contemplated anyway), or she will owe an additional 20% tax.
The most compliant/practical thing to do is probably to design the agreement to pay her in cash the amount of her tax immediately, and reduce the future payments. Most of the future payments, again, will be nontaxable.