OP, there are two separate aspects to DS; health issues and learning disability. The most severe health issues (heart and bowel) can usually be scanned for in utero, and the rest are pretty easily treatable. Even many of the heart problems are fixed with one op and the several children I know who had those were out of hospital in under two weeks. Seeing them swimming, tearing round softplay and trampoline parks these days, you'd never guess. My DS takes one tiny tablet daily for his underactive thyroid and has an annual check of his very mild heart issue. That's it. He's very healthy and honestly that's the norm.
The aspect you can't predict is the degree of learning disability. Will my son ever make it as an aeronautical engineer? No. Will he work, live independently, travel alone and have a partner? Almost certainly yes. Will he be a funny, companionable, compassionate, loving mainstay of our family? Huge yes!
There's been so much research over the last 30 years which has changed our children's futures beyond recognition.
When he was born I had never met anyone with DS before and I had a mental image of a non-verbal middle-aged person shuffling round the supermarket holding the hand of their elderly and exhausted parent. It's scary to me now how outdated my perception was. If you place high expectations on these children they will rise to meet them. People used to write them off thinking they were ineducable and that's so far from the truth. They do need to be taught in a way they can access (visual learning much better than auditory learning) but any school can easily do that.
I've just come back from watching my son at the front of the stage leading the rest of his year in a dance routine at the end of their Christmas play. Mainstream school. He can read and speak in sentences. He's six.
There's another thread on here from a lady who was in your position a few months ago and now has her daughter. You've probably seen it. Her updates at the end are lovely.
Here are a few more sites you might find helpful:
www.dontbesorry.net/wp/
www.oliverhellowell.com/
www.dseinternational.org/en-us/
www.dsmig.org.uk/
www.ds-health.com/