i am a prospective adopter and I have a wealth of knowledge and understanding of looked after children as I was a foster care worker for 15 years. Having looked after 60 or so children in this time period. i have birth children , So I think I am qualified to answer your question.
My birth children, are now in there 20's. I am very proud of them and how they have turned out. My children are all secure in employment and are in solid permanent relationships. My birth children have been brought up in a fostering household and at times I have unintentionally put them at risk.
I do not need to adopt to make my family complete, or to become a parent, I want to adopt because of my knowledge and understanding of looked after children and I still have so much to offer a child in order to get them to adulthood and beyond but i no longer wish to foster. Plus, i am FULLY aware of what I am letting myself in for. I Have a great support network, and my birth children will support me as I have already had the discussion with them that if I do adopt, thus them having another much younger sibling, then if something happens to me, would they potentially take on the child.
This is just my opinion and someone may well come along and advise otherwise but I am very much erring on the side of caution, when I read your children were 6 & 8, that immediately set off my alarm bells. So....
i presume that you have the most amazing , healthy, birth children that you love unconditionally and that life is good! No one has a chystal ball, But why would you potentially put this at risk?
can you bring another child into your family and you may not ever have the same strong feelings of love for them.
can you risk your birth children being traumatised by various behaviours displayed by a new child
can you risk your birth children being abused by another child? A bit dramatic, but it happens.
potentially what can happen is, you can "over compensate" with your birth children and that doesn't do them any favours in the long run.
I think you need to ask yourself what is your motive for adoption and is it worth upsetting your family unit.
And if you do decide that this is something you absolutely want to peruse, please do not go for a child that is close in years to your birth children.
in my opinion, you are blessed to have 2 amazing, I presume, healthy children, I would advise you to enjoy them for a few more years and then if adoption is still something your thinking about then look Into it.