hau Jen, the allopathy of sunflowers is minimal for most plants so I really wouldn't worry about it until you see signs of stunting and then, make notes on what plants seem stunted so you know what not to plant near the sunflowers.
For your raised beds (where you grew them) all you need to do is make additions of mycorrhizae to the
roots of the plants you install next, the fungi will do a lot of remediation of any allopathic substances in the soil.
I love that you are adding compost yearly, to get the dropped seeds out you would need to sift all the top 12 inches of soil in those beds, the screen needs to be smaller than a sunflower seed.
The weather of 2019 has changed most of the gardens in most of the world, either no rains that should have come, or too much rain in places where there usually was less rain.
In Arkansas we got our normal Feb. rains but the events were far heavier rain events than usual, and these rains continued up until the end of July where usually they stopped by the end of May or the first two weeks of June.
Almost all fields were saturated and remained that way long
enough that most crops were not in the ground until two months past the normal end of planting season for that crop.
The early crops like strawberries were poor because the plants were soggy from all the rains. Tomato vines rotted before they set blooms in many areas and others the blooms didn't show up until July or August.
This year should be an eye opener for gardeners and farmers because the next few years are most likely to be very similar to this year.
Redhawk