The other thing to consider about your OH's mobility is the level of pain management and sustainability that causes. It isn't just about his ability to care for the child, but also his ability to consistently support you too. You will be knackered and emotional and frustrated, and it can have a detrimental impact on your relationship, which in turn can impact on the stability of the house for the child.
In terms of finances, we figured we were pretty resilient, but it hasn't been easy. Financial pressure can be incredibly stressful on all relationships, especially if choices differ. We definitely upcycled and got some stuff second hand, but we were also mindful that some things the kids needed for consistency were the more expensive kind- we kept the same branded hot chocolate for example, and washing powder, which was a small but not unnoticeable expense. Add into that the costs of getting to therapy (3 times a week), needing to replace our car, school uniform, replacing dropped lunch boxes, shoes that were thrown out the window.... And some things you can get second hand or cheap, but some things you just need when you need them. And stressing about how to buy them on the cheap is, well, stressful. You always pay in time or in money. For example collecting the right sized clothes from charity shops or FB marketplace is all well and good, but it takes a long time, and isn't always guaranteed. If there is any way for you to avoid being reliant on that, or show you can accommodate for it, the better you will be. The initial set up for children was nowhere near as much as the ongoing expenses have been.
And then there is also little things like clubs and children's birthday parties, both hosting and attending, all of which cost money. Obviously they don't have to do those things, but if you factor in the social thinning adopted children often experience, anything that you can do to boost their social engagement is helpful. Again, it doesn't have to cost a lot, but it is a cost to factor in.
That's not even thinking about childcare costs when it eventually arises.
We basically saved up a year's salary, and then lived off my my other half's wage for the first year and then supplemented it by eeking out my wages plus savings over the two years that I ended up taking. We also paid off all debt apart from the mortgage.
So, if I were you, I would be looking at saving up an amount that you could realistically say to social workers that you could live off if you get to the end of adoption leave and you need more unpaid leave. If you have £1900 a month now and both of you can live comfortably whilst saving £100 a month, y@ou need to consider that it would take you 18 months to save enough to extend your adoption leave by 1 month, but only if your costs don't increase, and you have no other expenses.
Do you have family or friends that you could turn to for support if you needed it, for example? Could they support with childcare or even financially?
The final thing I would more is that whilst debt can be ok, you have to remember that it does rack up. So have you included the cost of borrowing the money in your calculations? Knowing you are spending £1000 a year for the privilege of owing a company money without even repaying what you owe can really focus ones mind!
There's no reason not to get the ball rolling though, and getting a view from social workers about what they think would be a decent amount to save. And you also have to remember, it is also about convincing the child's social worker, not just your own social worker.