Butternut Squash Plant help

Hello all, a few months ago an errant butternut squash seed escaped my compost and installed itself in my garden bed and has since completely taken off. I never expected it to get past the seedling stage but now it’s a flourishing vine throwing out new leaves and flowers on a daily basis! I’ve become quite invested in it and want to nurture it as best as possible in the hopes it will eventually bear fruit. Some important information for context:

Location: Perth, WA (a climate quite different from the North American origins of the squash)

Soil Type: really crappy sandy soil that dries out very easily but to which I’ve added some potting mix from Bunnings and some of my own home compost. I’ve also added in fruit and vegetable fertiliser with a slightly higher potassium concentration than generic fertiliser to promote flowering and fruiting.

Watering: was receiving twice weekly early morning watering from the reticulation and a roughly weekly deep watering on top of this. Perth has received barely any rain since last spring.

Sunshine: the plant is in a garden bed along a retaining wall down the passageway along the side of my house so is overlooked one one side by a high fence on one side and on the other by my house wall. I’ve guided 2 of the tendrils up only a trellis along the fence to get more sun by really it only gets 1-2 hours of direct sun per day and the rest of the time it’s all indirect.

Condition of the plant: growing rapidly and throwing out new leaves and male flowers that look really healthy. At one stage it bloomed a female flower that i hand-pollinated but unfortunately later that day i chopped off that stem by mistake while pruning some withered leaves. Unfortunately the leaves at the base of the plant have been developing white/grey spots that turn yellow, then browning at the edges and then shriveling away. I thought the spots were powdery mildew and I’ve been spraying the underside of the leaves with fungicide but it doesn’t seem to stop the leaves from turning brown and shrivelling.

Is my butternut squash a lost cause? I realise sunlight, soil quality and potential fungal infection are likely all working against me, but the lush new growth and daily flower development has given me some hope. Keen to hear some advice/experiences from people who’ve grown butternut squash in Australian gardens before.