Hi OP, I don't like Seren or Enfys either.
Mabli is currently popular and rather nice, as is Mali (Pr.Mally not Mah-lee).
Gwen is cute.
Non is lovely.
Manon works.
Alys is ok but will get Alice spelling.
Elin and Catrin are nice but nicer in Welsh accent.
I like Dona like the saint (Llanddona) and I think it's a name in a Daphne Du Maurier book.
I think Lona works and I've always liked it.
Avoid the letter combinations ow, ei, eu, ae, au, ai, aw, iw. ce, ci, ge, gi, ngh, wy, ar and probably others I've not thought of
Avoid f, rh, ll, dd, u.
Avoid -in, -en, -an endings unless you don't mind the anglicised pronunciation.
I've posted dozens of times I'm sure, but go for names that sound exactly the same in Welsh and English, and are phonetic in both languages.
You could look at granny names like Bronwen, Menna and so on.
Try the names out on impartial non-Welsh speakers who can say the names out loud to you.
Don't even go near names like Buddug, Dyddgu, Lleucu, Llinos, Ceri, Eirian, Rhian etc. It won't matter a jot if people can be taught to say the names - they usually can't, you'll be subjecting the DDs to a lifetime of " What? ", " How do you say it? " " Did-goo? Bud-dug? Clee-oocoo, Clee-nose, Surree/Cherry/Cherie? Air-yun? Ree-Anne " etc.