My 6 month old wakes up 2-4 times for a feed
It's a chicken and egg situation. While having night feeds then daytime calorie intake with solids cannot increase to the amounts baby needs, but without the quantity of calories in the day, baby will be hungry at night. So which to tackle first?
Definitely be conscious about the types of food you give. Use higher calorie fruit and veg more often (avacardo and banana for example), include meat and protein in the diet as much as possible, also carbs for energy and yoghurts for easy dairy.
Milk intake is going to need to be maintained at this early weaning stage. I was giving 7 milk feeds per day at this age, plus 3 meals. But all 7 of those milk feeds were in the daytime. The milk feeds do not need to be at night. They often are, but they don't need to be.
Swapping to daytime calorie intake is tricky. It's not physically possible to make a baby eat more than they want, so if baby is full from having over half his milk intake at night, it's going to be hard to get him to take more in the day.
Do you have an alternate way to settle baby to sleep that is not milk? I'd start at bedtime with separating feeding and sleeping, so give the milk feed before bath time, for example, not after. Then settle baby to sleep in the cot (with your help, I am not suggesting keaving to cry) without a feed. Then continue your settling method through the night.
picking him up when he starts to cry (30 seconds) and then put down on repeat
You are time limiting how long hes picked up for? I'm not sure that is the PUPD method.
I would have a ladder of increasing in-cot settling methods that end with picking up. If baby needs to be picked up and cuddles, I would cuddle until baby is calm and no longer crying, then repeat.