A bit dated as a middle name, but still nice. Wyn.
Usually a boy's name but used as a unisex middle name. It was very popular in the 1960s.
Gwen is sometimes used as a middle name.Means the same as Wyn (white, blessed), but is the feminine version of Gwyn, from which you get Wyn.
Evelyn Gwen Surname sounds pretty.
As does Evelyn Fflur.
Haf and Mai are quite meh as middle names. Pretty enough but used a lot.
Siân was so popular in the 1960s, and many girls were Siân Something, where Something was the name used day-by-day, so it strikes me as a bit odd as a middle name, but I love the name.
Maybe your name has a welsh version that you could use as a mn.
Non and Manon are nice but could be tricky if your surname is a noun (Hill, Foot etc).
Nest is beautiful but I'm not sure if it travels well. It's from the same as Agnes, but Annes, Annest and Anest are versions too. Annest would work best.
I like Mallt (Maud).
If you are looking at online lists, bear in mind that there are some crazy suggestions on there. You can get away with a boy's name as a middle name, something like Hedd would be ok.
The names suggested on here will tend to be middle-aged names or very recently trendy.