The name is OK. It should go with most names. Lowenna and Morwenna are Cornish not Welsh but I think they'd work and both are pretty.
Olwenna looks a bit like you really wanted Olivia but couldn't use it.
Single syllable names tend to be popular and the 2-syllable 1-syllable name pattern seems to be a bit overdone.
Popular short middle names seem to be:
Haf (summer-born daughter)
Mai (May-born)
Medi (September-born)
Wyn (1950s-1980 born boy or girl)
Fflur
Grug (heather)
Mair
Ann (a bit 1950/1960s/1970s)
Non
Gwen
Gwyn
Llwyd
Rhys
Siân
Siôn
...
Angharad and Eleri are popular middle names, but I'd be tempted to go for something more unusual like Mallt, Glain, Ffydd or Hedd.
I'd use a name you like or something like your grandmother's first name or a family name.
You could use the Welsh version of a name e.g. Mererid, Marged or Mared instead of Margaret, or Llwyd instead of Lloyd, Rhys instead of Rees, Fychan instead of Vaughan.
If your husband has a Welsh name or a name with a Welsh equivalent, you could use that as a middle name. (e.g. Sarra Elgan)
It's also not unusual to use a place name or river name as a middle name. (e.g. Bethan Dwyfor, Hywel Teifi Edwards )
A few Welsh celebrities use their middle name as their surname. (e.g. Bryn Terfel, Carys Eleri, Elin Fflur, Catherine Zeta-Jones)
Surnames as a middle name were popular in my age group. The middle name was often from the mother's side.
(My DC have no middle name and very ordinary first names but the surname was fairly unusual)