You don't necessarily need to retain a lawyer, but you absolutely need to have a few consultations with lawyers now, and ask them in advance that you want to talk about the future 50-50 scenario. Its either a very smart or very terrible move, depending on your jurisdiction. When you do consult, what you need to ask is if there is precedent for this in your area, how binding would a separation agreement be over time, and what is an appropriate and enforceable timeline.
The smart part of this is that she is probably thinking very short term and you can give up more short term for a better long term solution. The negative is that you are kicking the can down the road, no matter what she agrees to now, assume this will be a fight and the more time you give up primary custody and pay her support, the stronger her position gets to preserve this as status quo. It is often impossible to make any changes to a situation after a year. And she has made zero sacrifices so far - whether it is now, 6 months or 2 years it doesn't matter, she will fight any changes. And while you may think its better for the kids if she doesn't work full time now, I would disagree. I think its more important they get equal time with happy parents even if that means a couple more days of daycare.
So, I would look into something along the lines of at most a 6 month transition to 50-50, something like a 2 year limit on spousal support, and also write in long term expenses like activities and college should be 50-50. If you think its hard to pay the bills now, just wait 10 years. Also, I would give a deadline for her to sell or vacate the house and then you move back in. She will probably sabotage selling it for as long as she can since you are footing the bill, so pick a date based on how long you can afford knowing it wont sell until she vacates.
I know this is all scary, but continuing on in your current path was really the worst outcome. Now things are moving. Accept that it will get worse before it gets better, but know that it will get better.
