First, I don’t have a birth child so feel free to ignore me! I have three adopted teenagers - one almost 20 (wow!), one 17 and one 15. They all came separately but without - as you can see - huge age gaps. They were all 12 months and under when they came home.
As well as thinking about age gap, I’d think about where your birth child might be socially, emotionally and educationally at various trigger points for an adopted child - ime it starts to get harder when your child starts school and unless you have a school which wants to understand and work with you and your child then it can be tricky for all - particularly if a birth child is in school with your adopted child. My three attended different schools.
On the basis that no two kids are the same, my eldest was pretty much fine until 15 then he turned into a horror - common in all teenagers. He left for uni at 18 and is now a much nicer person!
Middly was a hell child from 3-11. During that period he was dx adhd and and had various school moves, all of which took a toll financially as well as emotionally. Doing well now and mostly a joy.
My youngest was probably at her trickiest from 8 - 12. She was dx asd/pda at 7. She also has learning difficulties. We’ve had some interesting times with her but now for the most part she’s a regular stroppy teenager
Right now things are going well. One in uni, one doing his A levels and one her GCSEs. It’s the easiest it’s been and we’re into year 20 so it can be a long haul
It’s doable and I’ve no regrets but there have been sacrifices and it’s been a full time job so go in with your eyes wide open, educate yourself as to the possibilities of future difficulties, read up about fasd as it’s a probability and just accept that it may be an entirely different parenting ride