There is no way to access a bucket using multiple CNAME record.
Your url must be same with GCP bucket name whether it is root or sub.
I think forwarding the url pierre-alexandre.io
to www.pierre-alexandre.io
is only option that works.
Refer here to how to use url forwarding on Cloud flare.
Or you can workaround to having secondary GCP bucket named your subdomain like www.pierre-alexandre.io
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…