Helo Lucy,
The Geiriadur Prifysgol Cymru has etymological notes for many entries. It is available in printed form, as a set of four volumes clocking at several hundred pounds, but I’d recommend checking out the website or app first
“Welsh Roots & Branches” develops vocabulary by building families of related words - not exactly what you asked for, but may be useful. A tutor I had with Dysgu Cymraeg thought very highly of it, but as it’s currently out of print it’s either hard to find, expensive, or both!
I have this, and it’s a fabulous book. Not necessarily one for systematic learning or research but just one to enjoy dipping into and browsing through.