welcome , sad to hear about fish loss . i was thinking before u can upgrade and have budget space . ur pond is more deep then wide .. like a box , it shud b more like a bowl or plate to give bigger space for fish to swim , u can easily increase its surface area for there swimming by reducing depth to 24inch , if u have extreme cold then 30inch , that way u will have more liner for the width and length it wil increase more surface area for gas exchange and sun light , more algae food , more oxygen , more space for swimming bigger viewing area , improved visibility till bottom , more bacteria as they grow more near surface , if ur pond was 12ft wide then 4ft depth wud b proportional .
other thing u can do is add additional bog or filter as sisy suggested a place fr additional water and filter media which will keep water quality good , have strong pump with aerator and have strong current then u can increase more fishes easily .
if ur pond is around 1000 then have atleast 2000g/h pump , more is better u can use it when u upgrade ur pond .
i think ucan easily have 3 to 5 koi with goldfishes as they will not grow 24 inch in a year , if u hv fry they will only reach 8to14inch 1st year then next year 3to 5inches and so on , they will reach 24inch in 2 to 4 years , it also depends on quality and quantity of food , if some one is overfeeding them , they may grow faster and also in gene if jumbo koi or females will grow faster larger . butterfly koi grow sloww and they dont become very huge often . untill then uil upgrade pond .