I can tell you what I did to beat but not eradicate a combo of amphidinium and ostreopsis. I can also tell you it seems like there is no one size fits all solution but a laundry list of possible things to try. Don't get discouraged.
What I did:
My corals were being smothered, dinos had a grip on every inch of the tank
-Installed a UV (didn't feel like it helped much but can't be sure)
- Daily testing and dosing Neophos/nitro accordingly
-cut my lighting period by 2 hours and turned down whites/greens/reds to basically 0, also cut intensity of blues by 25%
-alternated dosing Live Rock Enhance and MB7 daily
-i scrubbed the rock and siphoned the sand daily the best I could, sometimes twice a day.
-cut plastic cutting boards to fit around my rock work to shade the sand. Held them in place with zip ties on magnetic frag racks.
-ordered the Jar package from Tampa Bay Saltwater (arrived after I turned the corner on dinos)
I no longer have any bubbles or strings on the rocks at all. My corals all made it (barely). After I got through it, the tank almost had a spring like event period. Things started appearing like pods, baby trochus snails, etc. Really cathartic after what felt like a near tank-death experience. Now I still have a few lingering but manageable areas of amphidinium on the sand. I received the TBS package of sand and rock which I have in a separate tank for observation. I added the sand as a new layer on top of my display sand a couple days ago. I'm waiting on adding the rock which I will soon after I confirm there aren't unwanted hitchhikers. Hoping the real ocean sand on top will help keep the amphidinium at bay long term.
I'm no expert, but it feels defeating dinos is purely guess and check until you find something that works for your tank. The good news is the resources of all of the things you can try are readily available on here to read through and gather all the info you can to put together an action plan.