Interest only mortage and investing the savings over repayment - tell me I’m going nuts

Disclaimer - im aware of the risks of the stock market and changes to inflation affecting these sums

A random thought came to me last night while considering my mortage - is getting an interest only mortage and then committing the money saved over repayment to stock investment a smart idea?

Heres my sums -

£300,000 mortage over 30 years

Repayment at 4.5% - £1519 a month

Interest only at 4.5% - £1124 a month

Investing the difference (£395 a month) into a stock investment with a conservative average return of 8% over 30 years - £596,143.

Have I not instead just paid my house off in full AND made an extra quarter of a million on top? What have I missed?