Thank you Ma'am
The short answer to your question "can you change the separation to a divorce" is yes...that is possible.
That is, it is possible to modify the filing with the court...well, it is really just filing a new petition to divorce to replace the petition for legal separation.
But the thing is, you will need a lawyer to assist you on this.
While a legal separation requires the parties agree on the terms of the separation (you can not do a "contested" legal separation), a divorce can be either contested or uncontested
Uncontested; Now the fastest, (and by far the least expensive) way to get a divorce is if you can agree on the terms. If you can agree on "who gets what" there is not a need for attorneys...this will cut the costs to a very small amount (court fees)
Contested; if you can not agree, this is a contested divorce. To proceed you need an attorney or need to act as your own attorney (very bad idea). Here the parties present evidence to the court on "who gets what" and the court decides. This take longer and involves attorney fees for both sides.
So what you describe? It sounds like you are close...but not there with custody.
And that is a key piece. Since part of the divorce process will require the court to issue a custody order, and likely a separate support order.
IF you can agree on custody, then it is easy. So long as it makes sense...so long as it is reasonable and you agree, the court will approve. Support is a bit more complex...since there are formulas the court will look to in deciding support...but if you recommend a support amount to the court they will consider that and may approve...or may modify.
But if you can not agree?
Then you must present evidence to the court on why you should get the time with the children you are requesting. This requires a lawyer to prepare and present. You can try this yourself...it is called pro se
...but it is a bad idea . The adage "a man (or woman) who represents himself has a fool for a client" is often true. If you act as your own lawyer, the court will expect you to know the law/rules/procedure/evidence just as if you were admitted to the bar. They (the court) will not cut you any slack.
You may want to offer him to give up the majority of the property in return for agreement on custody. IF he agrees, you can file the divorce yourself.
Otherwise you would do well to have a lawyer assist you.
Let me know if you have more questions...happy to assist if I can