Have you tried the 'Find Professionals' tab at the top of the screen under the Houzz banner? You can refine for various options but not sure you can filter for design-only services

I'd also recommend checking out the local chapter of the APLD (Association of Professional Landscape Designers). This organization is as close to a national certification program for designers as there is and many (most?) are happy to offer standalone design services for DIY'ers. Here's a link to the NE chapter: https://www.apldne.org/find-a-designer-2

I'm not sure what his service radius is, but Andrew Garulay ( https://www.yportdesign.com/ ) is pretty awesome.

http://lilliannewbertdesign.com/index.html

Know that a high quality design will come with a commensurate price. Often, homeowners who aren't familiar with professional design services are surprised at the scope of such a project. It's not just a matter of selecting a few appropriate plants. Landscape design, at it's best, is an art and a science and can be quite involved. Lillian was an acquaintance at Radcliffe and does beautiful work.

Matthew Cunningham. He's pricey, but really good. Boston to Maine. He's on Houzz, look him up.

https://www.houzz.com/photos/query/matthew-cunningham-landscape-design/nqrwns