Going back to the flowers you like, let’s divide them by categories.
Hyacinths, narcissus (daffodils), tulips are spring bulbs. This means you plant them in the autumn, they need cold temperatures over winter and will then grow the next spring. If you pick spring bulbs carefully, you can have flowers from late January until spring, and that’s very cheerful when the garden is otherwise bare and sad. Some will come back for many years, others will only flower once. Since this isn’t the season to plant them, let’s park them for now and we can revisit in the autumn. Although note, you can buy bulbs “in the green” right now from supermarkets and garden centres, and put them in your planters so you’ve got a display before planting annuals in a few weeks. Pansies, although not a bulb, can also be planted in the autumn alongside bulbs, and they will flower over winter and into spring, until you replace them with your new season bedding or planting.
Hydrangeas, roses and lavender are perennials. This means you plant them once (preferably in the ground), and they will grow bigger and bigger every year. Roses love sun and clay soil, lavender needs sun but prefer dry conditions and poor soil, hydrangeas do well in part shade. They are not necessarily plants that grow well together. I would suggest you think about setting up borders that will incorporate these plants. Because perennials can live for decades, it’s worth pausing to think about the structure you want in your garden, and not rushing to plant them now. Having said that, most perennials can be moved if they are not happy, so don’t be afraid to try different things. I move plants around in my garden all the time.
“Geraniums” commonly refer to two different types of plants: hardy geraniums, which are a perennial and will spread and grown bigger year on year, like geranium Rozanne https://www.bethchatto.co.uk/shop-by-other/plants-for-containers/geranium-rozanne-gerwat.htm, and pelargoniums https://www.jparkers.co.uk/120-geranium-f1-mixed , which are treated as an annual and do well in pots (although you can store them in a shed and bring them back the next year). Which one do you like?
Petunias are annuals. This means you buy plugs in the spring, plant them out around May (depending on where you are in the country, could be April in Cornwall or June in Scotland), and they will thrive for one summer, but die in the autumn and need to be replaced every year. They can grow really nicely and fill out a lot of space quickly, so are ideal for large planters like yours. Most of them need deadheading, but there are some varieties like Vista Bubblegum that are sterile - this means they don’t produce seeds, so don’t need deadheading. This is particularly relevant for petunias because the flowers are very short lived, so if you don’t buy sterile ones, you will need to dead head every day (or every other day) or they will stop flowering. Some people enjoy deadheading - it’s easy and meditative, but if you won’t, select a variety that doesn’t need it.
Your planters will need to be filled with fresh multi purpose compost, mixed with some grit or perlite to help drainage, maybe some water retaining granules, and slow release fertiliser. Then you should top this up with more feeding throughout the growing session - something like blood fish and bone or liquid seaweed in the spring, then Tomorite or Grow more from June onwards. Tomorite (or tomato food) encourages flowers and fruits for all plants, not just tomato plants. You should aim to feed every other week or so, or every month depending on the type of fertiliser. There are loads of YouTube videos that can help.
When planting containers, it’s good to combine three different types of plants - a thriller, a filler and a spiller. Your thriller is what adds height and showiness, the filler is what gives volume, and the spiller will cascade down, perhaps adding greenery or a different leaf shape. Again, there are tons of YouTube videos and online advice on plant combinations. Repetition is very important aesthetically, so it’s better to have fewer varieties but repeat them, rather than to have lots of odd plants.
I appreciate this is a lot to take on, but please come back if you have any questions.
And yes there are lots of other flowers you can grow from seed outdoors - but petunias aren’t one of them 😊
Also petunias need a lot more moisture than pelargoniums, so I wouldn’t put them together in one planter.