I'd tackle it all at once. When I take Antibiotics , I always also eat yogurt with active cultures to keep up my "good" bacteria. In your case, I'd take extra lactobacillus supplements while attacking the bacteria with Antibiotics .
Can't prove it's true, but Humaworm claims to cure blastocystis, or it used to. You might check their site. That being said, if I was that sick, I would take prescription Antibiotics to get the blasto under control, because that's serious stuff and needs to be eradicated immediately. After 4 years, it will take a long time to rebuild your health anyway, so that will be an ongoing process.
First, get rid of the blasto. Nothing will help you until it is gone.