I won't give my opinion. You can only get actual legal advice or an opinion from a lawyer that is your own lawyer. Lawyer's on this site cannot provide legal advice as it is unlawful to do so online and when the customer is not your client.
There are pros and cons to paying a lump sum.
The pros are there is certainty, there is a clean break, there is no returning to court or mediation when things change.
The cons are that there is no tax break (when there is a periodic payment you would deduct all payments from your taxes for tax purposes, if your spouse dies soon you have paid too much, if she marries soon you've paid too much etc. As well, she keeps the money even if she gets a better job and earns a lot more etc.
You most assuredly need to have a lawyer do the separation agreement and you both need independent legal advice or the contract may not stand up in court if it is challenged. A mediator can help you come to an agreement and even help draft it but you would both still need to get independent legal advice.
We rely on divorcemate and generally consider the mid-range.
If you each have the children half the time then you would pay less support as you would take what you would pay if she had the children full-time and subtract from that what she would pay if you had the children full-time. If the support is periodic then that spousal support would be included in her income and so she would pay more child support and pay more toward the section 7 expenses as well so that is another con to the lump sum payment.
Does that help?