I don't know what to say to that. I don't want to sound negative, because there is a chance that it will work out as you hope. But it is a bit like taking a deep breath holding hands and jumping into an abyss with no parachute.
The highest risk is that the marriage breaks down, triggering legal fees at a point where you are also juggling school fees and after school care, without the additional support you are hoping marriage will bring. The legal costs thus incurred have to be less than what you need to pay currently, to make it worthwhile.
That is before you consider the emotional costs of having blended a family, for it to then split up.
Honestly, I think, taking love out of the equation, you are living beyond your means, and this is a business arrangement you hope will allow you to continue doing that. It is actually not your problem if you are further away from xH because you need to downsize to be able to manage your life. At least part of the reason you need to downsize is costs and conditions imposed by his family.
That apart, looking at the practical gains you hope to make by marrying, have you discussed these with your dp? For example, what help you need, how many hours you need him to be at home with the kids, or to do house-keeping, jobs around the house etc. Have you seen evidence of his willingness to help? FWIW, I am a single parent and I have a lady come to the house now for a couple of hours three afternoons a week, plus two mornings, in addition to my childcare. But I live in a small house, so it's the choice between managing my life in a small house or moving to a bigger one which would give us the space we need, but also give me more work and mean I couldn't afford the help. That's my choice and I am not saying it is right for everyone, just that there are different ways around the problem.
But as your heart seems set on marriage, I don't think anyone here will dissuade you. I do think it is interesting that love is the last point you make. I always wonder what love means. In the context of marriage, I think it means being able to imagine, even plan, growing old together. When I stopped seeing my husband in the cottage I visualise for my dotage, I knew things were on the rocks.