I'm in the fuck no camp.
So let me get this right.
He will hire you through his company. You will then use your 'salary' to contribute fifty percent towards the household expenditure, so fifty percent towards the expenses incurred for nine people, compared to you spending on three people currently? Your salary however will remain the same as you earn currently? Is that correct?
You will be doing the all childcare of seven children, probably cooking as he doesn't cook. Spending precious time with your own children is very different to running around after seven children, especially when five aren't yours.
Will he pay you a pension?
When do you get free time from seven children?
He managed to get fifty fifty child contact despite his wife having been the main carer. And you think that's wonderful. There is no way on earth a man who is a millionaire, has access to the best legal advice, was unaware that his child maintenance contribution would be zero if he had the children fifty fifty. My DC's father found out swiftly and attempted for fifty fifty childcare too.
The DC get on really well currently, but they're children, children fall out all the time, what if in the long term one or more child decides they hate your DC or your DC don't get on with theirs. What will you do?
Also when you move in your DC will quickly become aware of the financial disparity between their lives and that of their step-siblings. This will lead to upset.
You will be in in charge of childcare for seven children, this is totally different to spending time with your two. Will you have time with just your two?
If you marry him, and if the marriage does not succeed, you will need money to get a good divorce solicitor. You'll be limited by the salary you are paid by your DP, and the money you have left after contributing fifty percent towards the household expenses.
And a marriage that fails in a short time (I think it's under ten years), you will get very little, I also doubt very much a man who is as rich as you say, and has been divorced once and owns his own businesses, won't have had legal advice on how to ensure he does not lose his assets on divorcing a second time.
Your DP's ex got a good payout as she was married to him for a decade and had five children with him.
If you really want to go down this rabbit hole. You need to be very clear on what he expects of you in terms of childcare, housework, financial contribution. And also ask yourself, will you feel able to tell him no or refuse to pick up additional household drudgery if he is paying you a salary and putting you up in his house? Or will you feel you 'owe' him for housing you and your children?
This is nothing like some posters saying well fifty years ago my DH asked me to give up my job and marry him, yes they weren't expected to give up their financial stability and independence in exchange of taking care of five children that weren't theirs and paying half towards a household expenses where the majority of the occupants were not theirs nor the house in an area of your choice.
And when will these house renovations take place, before or after you move in, will you be paying towards these house renovations too?
On the other hand you are looking to give up financial independence, pull your DC out of a school which is excellent and they are settled in, leave behind all friends and support network for all three of you, throw your only financial asset your house into a marriage and risk the possibility of losing half in a divorce, also if you put it in your DC's name, what about if you need to go back to it in the event of a divorce, then you still won't own it as it's your DC's and then what if they get older and want their share?
Get legal advice.
What's wrong with just continuing as you are and you both continue parenting your respective children?