Questions about Google Family Link and WTF is it so complicated?

Hi, general questions about family link and a few specifics.

My kid, 12, just got a Chromebook, more online school needed and it behooves him to have a separate account for school but other things like scouts. Discovered Family Link seems like a great service, but apparently it deletes your stuff and it's not coming back.

I have the wrong birthday for my kid, apparently once linked that can never be changed. Ok.

Last night, I set his child account as a the primary one on his chromebook. He needs access to youtube, which is helpful. Impossible since birthdate can't be changed for years to allow access.

That's annoying, but here's where I'm livid. Once account was linked, it apparently deleted all his emails. Me, wife, have sent random stuff to account we created about 10 years ago, just as a surprise. Pics, little stories, etc. It was there yesterday. After I linked accounts, it's all gone. Makes me want to puke.

TL;DR, is it correct you have no flexibility with linking google family accounts and it appears to just delete your shite? Any idea how to help that?

Thanks