Chinese Overstay Query

My partner is illegal Chinese.
He's overstayed his visa by 6 years.
We have been together for 2 years.
We'd like to get married but unsure the future and legalities and risks.
Is there opportunity to declare intent to marry and apply for spouse visa? I meet the income requirements.
What are the general percentages of failure for the above situation?
We can prove genuine relationship if it got to interview stages but anxious. With how little percent of illegal Chinese do get deported is it even worth the risk involved to try to go the legal route?

You will not be able to be legally get married in the UK, since he is not legally allowed to be here. There is no way around that, he will need to go back to China before you can do anything.