Reduced joint mortgage for sole income

Fed

My girlfriend and I are at the beginning of the process of looking for a property and are trying to work out what we can afford.

We intend to purchase as tenants in common with me paying for my share of the property in cash whilst she will get a mortgage. Now as I understand it the vast majority of lenders require us to apply for a joint mortgage as we both want to be named on the trust deeds.

We have a deposit for 80% of the property and hope to get a mortgage for the remaining 20%. This 20% is roughly 4x her salary and seemed feasible from calculators we used and a broker we had a consultation with.

The issue is I make a living from gambling (have done for years now) so as far as an application goes my income is 0. The broker we spoke to didn't seem to think this would impact the application or the amount we could get and that view was echoed by some posts I came across online. However we went to Nationwide directly and they ran the numbers and adding me to the application reduced the offer from 4x to 3x salary, I guess as i'm viewed as a dependent

Is this common practice amongst lenders to reduce the amount significantly if one applicant is unemployed?

Is this something brokers can get round? For instance if we went through a broker to Nationwide might we get accepted for 4x?

Does the fact I have a decent amount of fund investments (which won't be used for deposit) help in any way?

Any other options we could pursue?

Thanks