Hello! Welcome to the forum. My plumhead, Pi, is going to be 6 this year. They are wonderful birds.
I would prioritise fresh veggies and some fruit. I also personally sprout seeds for my Pi. Plumheads will accept a wide variety of fresh options, especially when they are young. Silverbeet, green beans, carrots, sweet potato, and zucchini are pretty popular in my house. For fruit: passion fruit, dragon fruit and mango are well liked. Plumheads natural diet includes flowers. They love flowers, but only feed food quality flowers - from a grocer for human consumption. Never use flowers from an ornamental bouquet.
Plumheads can be expensive to feed because they do require fresh produce. However you can save some costs by buying frozen mixes. I do this for dragon fruit for example when it is out of season.
I would look at what is in the seed mixes and make a more informed choice. For example some cockatiel mixes contain sunflower seeds which are not ideal for captive birds. I generally feed a budgie seed mix because in my area the seeds are a better variety.
Happy to share any more information. Just ask