phytophthora and dahlias
As is sounds like you already know that Phytophthora thrives in poorly drained soils, looking at ways to improve the drainage will help whatever you decide to plant next.
As for your specific question about planting dahlias, this article from The American Dahlia Society suggests that you may also encounter problems https://dahlia.org/docsinfo/articles/fungus-control-for-dahlias-and-the-garden/ but they are listed as safe in this list of tolerant plant species https://plantpathology.ces.ncsu.edu/wp-content/uploads/2013/05/AG-747_Crown-Rot.pdf?fwd=no
My suggestion would be to see if you can improve your soil drainage by incorporating organic matter or by using raised beds. You may find this document on soils to be helpful http://cru.cahe.wsu.edu/CEPublications/EM063E/EM063E.pdf
It should work, but you can also use a search engine to look for the bulletin number. Try searching for: WSU EM063E