OP you’ve received a lot of alarmingly wrong advice here.
Your assets have not automatically become your husband’s. However, you have financially maintained him for a long time. The starting point in an inheritance dispute between a husband and his wife’s estate is the 50/50 split that would occur in a divorce.
You need to see a solicitor. Under the Inheritance (Provision for Family and Dependents) Act 1975, the standard for a spouse isn’t what is reasonable, it’s what has come before. Even a life interest has been held to be not enough.
You can write a statement to accompany your will, but it would need to be incredibly detailed.
His mental health issues etc., will also work against you. He can’t work, you’ve maintained him, and left him with nothing. In the court’s eyes, that would be held to be a pretty unreasonable act and it’s likely he would be awarded a substantial sum.
Yes a challenge can be expensive, however in a case like this it’s pretty cut and dry that there should be some sort of settlement - so he’d likely get a conditional fee agreement and the estate would end up paying his fees. If it went to court, the settlement would be even larger and fees settled as ordered.
Your best bet would be to go and see two solicitors - one who specialises in inheritance disputes, and a family solicitor. Go and find out what your best options are - is it to divorce him now, or wait on the off chance he won’t launch a claim?
It’s totally possible that he won’t. But as you’ve been financially maintaining him for a long time, any claim he did make has a good chance of being successful.