Dill is good but it can get fussy in the vase and droop, so cut some put it in a vase and see how long it lasts indoors. Those are great sunflower choices. Buperleum is supper easy and florists already buy it from suppliers.
I remember my local florist put in some Solidago she had brought in from a supplier, it grows wild here, a customer phoned her and made her come and take out the ‘ditch weed’ in his bouquet, he was upset so watch your choices and know the customer. Some don’t mind, others are offended when they see something they think you got for free in their arrangements.
I have never heard of Bupleurum, I had to look it up- that is a neat looking plant. Looks like a good filler. It reminds me of Euphorbias.
I can see your point about using natives, or other locally sourced ‘weeds’. People probably don’t want to spend money on things they can go pick themselves. I will check the dill and see how it does. I did that last year, but cant remember if it stayed fresh looking for very long or not!
I found that sunflowers did not benefit from pre starting. You are in a warmer zone than I so you might want to both direct sow and pre start some varieties. Those flowers that are not day light sensitive, meaning they kick into blossom mode only when the days grow shorter, can be pre started for earlier blooms and then again later for a later crop by direct sow. I did that with bulbs like lily and glads, that way I did not need to throw away unsold blossoms and waste my money.
My colder zone made it so I had to pre start most of my seedlings.
You might be interested in this channel on YouTube dedicated to growing flowers. She does a lot of videos highlighting each flower and how to start them from seeds, etc. It’s a “smaller” channel, as in she doesn’t have a TON of subscribers, but a good “hand full” and I really like her videos.