"Vet wants us to reduce his food intake by 30% but that seems quite a lot given weight loss is meant to be slow?"
I have never seen a healthy cat (or dog) dieted and lose weight too fast. Like, never. Ever. In 20 years as a vet.
Presumably he is neutered.
And sleeps indoors (and has fur unless hairless variety) , so needs to expend hardly any calories keeping warm.
And doesn't go any further than the enclosed garden, so his exercise is negligible. I do not really count pratting about with a feather on a stick for 10 minutes- it amuses them and is good for their mental health but calorie-wise is like me counting put the bin down once a week or emptying the dishwasher as exercise.
So you have to rely on cutting the food back. Hard. Have a look at the back of the packet but bear in mind pretty much all the food companies, even the good ones, err on the side of recommending too much (sells more food and saves that one lawsuit in America where some dickhead ignored the state of the dog and robotically kept feeding the amount recommended and that one in a million dog with an unusually high metabolic rate/ calorie requirement got thin and died. Or that is what the Hills rep once told me).
Weigh the food out, feed him for about 3.5kg in the hopes of reaching 4 or 4.5 or whatever he's meant to be, and weigh him every month, then when he is the right weight increase his food a little and keep weighing him. Simples.
I think "losing weight dangerously fast" is a very overstated risk. Most wild mammals in temperate climates would be very very lean at the end of winter, for example- they can cope with fluctuations. Hepatic lipidosis in uncommon and tends to happen in obese cats that are completely anorexic due to illness, the illness that caused the anorexia placing a lot of oxidative stress on the liver anyway.