I would consider Wm for primary if I was a long-termer in Wales.
BUT if I were to do this, then I would have to be prepared to get really serious about supporting my child's English education outside of school.
I live in a non-English-speaking country, and because international schools are expensive, most of us English-speaking mums educate our children in the local vernacular.
However, you really do need to be organized about teaching and practising reading and ESPECIALLY writing skills. Tutoring at some point may be a good supplement.
And you need to work really hard to shower your kids in English vocab. I read three books to my daughter a day, have maps and educational posters around the house, point out new things and new words all the time, and am very proactive about doing lots of visits to science museums, planetariums, aquariums, botanical gardens and all that. They need to get a strong vocabulary in the home language from their parents if they are not getting it at school. Don't kid yourself that they are going to get a full vocabulary just from everyday "have you finished breakfast?" "it's time for a bath" and so on.
And don't kid yourself that they will get good fluent writing skills just from speaking the home language casually on a day-to-day basis.
Kids at WM schools in Wales seem to do no worse than those in EM, but I suspect that that is largely because the intake at WM is more middle class, and because... I'm sorry to have to say this, but educational standards in Wales are generally considered to be pretty poor, so the standard that kids from WM need to achieve to be considered normal/typical, is not very high.
I'd probably prefer EM from secondary school onwards.
In short, school-based bilingualism can be great, but it works best when parents are organized and motivated about actively teaching the home language.