I think what OP is saying:
OP gets
4 years of living rent free (mortgage he paid)
to keep her pension
50% of property value
£20k
OP has been paying for ... all medical & dental for the kids for at least 4 years (how much did that cost?) What about other kid costs in last 4 years? Who spent how much for kid shoes, clothes or transport or to go on holidays, hobbies, gifts?
stbX already paid
£700 4 12 = £33,600 in rent for last 4 years that OP did not contribute to
160 4 12 = additional maintenance for the kids, what did that cover, really, £7700
stbX gets
50% of property value
100% of business, value of which we don't know
We also don't really know what the business was worth 4 years ago, when they separated. That's the business value that should apply imho. And who has paid household maintenance in last 4 years, property tax, etc.
The kids are only teens, so NOT at Uni and I guess OP is saying there are absolutely no loans or any other way for kids to go to Uni if he doesn't pay. Presuming any of them want to go to Uni and he wouldn't support them to train to be plumbers or whatever, instead. Still, they will be adults then and it should be up to them as adults to negotiate with their dad for his support for their Uni costs.
Seems to me like could be a good deal for OP but real question is how will the kids be supported. What does the law in OP's country say about how much he should support the kids, and just how old are they? Why not agree to that whole deal on condition he has the kids at least 50% of the time, and there is a joint fund you both contribute to for the kids ?
Is how we are managing for our kids, we have a shared costs spreadsheet.