Hi op
To answer you original question. My two DS are in a Welsh Medium Primary, and my DD is in the local Cylch Meithrin, I'm English non welsh speaking and DH has some very basic Welsh. All my children are thriving in school, they have not been at a disadvantage at school because of our lack of Welsh at home, nor are they at any disadvantage in their education for learning their lessons in Welsh at school.
DD Is 3yrs and at nursery can follow basic instructions in Welsh, knows colours and numbers up to 20. And can answer very basic questions in Welsh. Obviously she does all of this at home in English.
DS2 5yrs is in reception and loves school, his English phonics are way behind his peers who are in an English school, but as I witnessed with DS1 when they start reading in English age 7, they soon catch up and are able to switch easily between the two languages.
DS1 Is 9 and has the English reading average age of 8/9yrs despite only being introduced to spelling, punctuation and english books in school at age 7. He is also now completely fluent in Welsh, enabling him to talk to people in the corner shop our Doctor and local landlady
in their first language.
In the wider world, Welsh probably will not be much of an advantage to my children. If they choose to live in the area we are in, it will be of help to them. However, I feel that an additional language no matter what it is can never be a hindrance surely?!