I'd be happy to review the letter that you write, but in terms of verbiage to use there is no legal language for you to use here. That is because, legally speaking, you have no law based argument to obtain what it is that you are asking for.
In employment law, unless you have a contract of employment guaranteeing that you can not be terminated without cause, your employment is "at will" and can legally be terminated at any time. Furthermore, severance is not legally required in any state, again unless you have an employment contract guaranteeing it (and even then, it would have to guarantee a certain rate which you aren't getting and are arguing for).
So, this current severance offer is not legally compelled against the employer. They are giving it to ease transitions and to obtain a release of any claims, just to make it a clean break. In terms of non-contractually required severance agreements, it is actually very generous compared to most that I see right now.
Negotiating for a greater severance, when the initial severance isn't legally required, is an extremely difficult and potentially dangerous thing to do. It's difficult because, without some contractual argument to support your claim you have to some the legitimate threat of a lawsuit. In this case, you've mentioned that you are in a few protected classes, but at the same time, you are being replaced by your former boss...so the employer has a solid argument against discrimination as the basis for their decision unless you have other evidence beyond what you've stated here of a discriminatory motivation.
The reason that I say it is dangerous is that, in contract law, when you make a counteroffer, you are legally rejecting an offer. That offer is then off the table unless they decide to put in back. I have seen, numerous times, a person counteroffer on a severance agreement only to have the employer reject the counter and then take the entire severance agreement off the table. The person then ends up with nothing, because they can't force the original offer back to the table and severance is not legally required (and those people didn't have any legitimate basis for a lawsuit that would force negotiations). So, unless you've been invited to negotiate this with them, I would seriously caution you against rejecting their offer unless you have a legitimate basis for a lawsuit that will keep them at the negotiating table.
If you DO have facts and circumstances, beyond what you have mentioned here thus far, which indicate discrimination based on one or more of the protected classes that you are in (race, gender and age), then your verbiage needs to make it clear that you consider this to be a termination directly connected to one or more of those categories and that you'll only be willing to waive your right to suit if you get the severance that you are requesting. This is a serious gamble, because if you don't have good enough facts to win at trial, you are rolling the dice on the employer fearing the lawsuit enough to negotiate with you, and most are willing to force you to go through the EEOC and federal lawsuit process before they are willing to negotiate.