Oh we have another polish member, maybe she could give advice for where to get some bird stuff?
@Rozalka
(Also I'm going to info dump a bit but I promise it will be useful)
If possible, you should introduce more types of vegetables. My advice is always 3-4 green vegetables(some of which should be leafy vegetables like spinach or romaine lettuce), 2-3 orange and yellow vegetables(carrot or bell peppers work well), and 1-2 red (beetroot, red bell pepper). Or just 6-7 types of vegetables(mainly green). You can even add something extra such as pumpkin or cauliflower. You can use any vegetables as long as they're bird safe. The reason why I state colours is because different colour vegetables often have different types of nutrients. Types don't matter that much, it's just the nutritional content. If that's too much to get at once, just change the types you feed every week.
Also, reduce the cucumber. It's not unhealthy but it's very watery so it might cause watery poop or overhydration(again, it won't kill him, but if it's the base of what you feed them it might cause issues)
It might seem like a lot of effort to feed so many vegetables, but don't worry. Here's what you do:
Get a food processor(not blender), throw in the vegetables and blend them until they are the desired size. You can chop by hand if preferred, it might just take more time. Then put the mixture into ice cube trays and let them freeze. And whenever you need to feed him, just pop out a cube, let it unthaw, and it's ready to serve :] they should last for about 2-3 months. you CAN microwave it to speed up the process, but it kills a lot of the nutrients, so unthawing it is better.
Anyway, sorry for the info dump, just had to make sure you knew everything.
tl;dr: provide variety(at least 6-7 vegetables). You can chop them up, freeze them in ice cube trays, and feed when needed. Cucumber is healthy but don't feed too much because of high water content