Hey Julia! this is a great question. First, I start so many seeds indoors because our season is so short here. With your growing zone, you have a much longer season, and thus way more opportunity to direct sow successfully. So to answer your question, direct sowing success is dictated by both the plant and the growing zone. Some plants are know to direct sow better than others.
You have a lot of space to grow in! But to keep things easy I would try and plant lots of cut and come again flowers; my favorites are zinnias, cosmos and dahlias, all which like to be direct sown. With those three flower varieties you also don't have to worry about planting multiple successions, which is nice if you feel a bit late on things time wise! Poppies also love being direct sown (but probably not Icelandic)
Dahlias and poppies are great focal flowers
zinnias and cosmos are wonderful secondary flowers
oh and calendula! that works well as a direct sow + secondary flower
another bloom that loves direct sowing is Nigella, it has fantastic texture, so it can work as filler. I would succession plant this one, doing a few rows every 3 weeks.
sweet peas you could plant out as a direct sow, those go out roughly a month before the last frost
Larkspur can also be direct sown, but you probably want to leave the seeds in the fridge for about 7-10 days before planting out. this would make a good line flower!
I would focus on 5-7 flowers this year, direct sow them, master them, and then next year add a few more. It's easy to overthink everything, when sometimes just tossing a bunch of zinnia seeds into the earth provides amazing flowers!
If you plant the varieties I've listed above, you will have great flowers the whole season through. I would just narrow down what colors you would like, order the seeds, and plant out in 3 weeks! You've got plenty of time.