There are so many acceptable variants in the Welsh names, and so many unacceptable variants but nevertheless common, that what we do is put whatever one we've lit on in the name field, and all the spelling variants, both Welsh (acceptable) and Englished (annoying but necessary) in the nickname field.
I especially like Margred because it reminds me of Terry Pratchett's character Magrat Garlick: https://wiki.lspace.org/mediawiki/Magrat_Garlick :-)