I (38 m) want to tell my fiancee ( f 27) her wedding dress choice is way too extravagant and suggesting alternatives
How is she going to react?
She will cut me out of her life.
That seems a bit excessive on her part but you also have to understand that weddings are an important milestone in people's lives. They want these moments to be reflective of the person they are and really express themselves. At the same time as the groom you should be able to do the same and if that conflicts with her views, you really need to talk to her but in an approachable non demanding manner.
She's being unreasonable here.
It is a pretty extreme response. If you're marrying this person and this is the type of response you anticipate in the future you might want to think long and hard about who you think your ideal mate is.