The question is, is dosing Si the current best contender because diatoms are the best competition against dinos or is it that sterile systems that can support dinos cannot support much of anything other than dinos, cyano and diatoms. I guess in either case for me the answer is to increase the diatoms but then I still will need to add something else to not revert back to the dinos right? My current plan is to dose microbacter clean and microbacter 7 while syphoning out and filtering out as much of the dinos as I can, and dose nitrates and phosphates (and try to find some sponge excel) until I can see the dinos losing ground (if they do) and then add some coral or rubble to try and introduce some biodiversity.It's even more amazing than that. These Large Cell amphidinium are a ghost in the scientific lit. They barely get a mention, they never get nailed down by species, and there isn't a single paper I'm aware of that talks about a bloom in the wild of this type.
Si dosing is a good move though, especially in your situation. Your system is overall fairly happy at the current nutrient levels, so IF you adjust them, I wouldn't adjust them much. Si dosing creates a strong competitor for the exact same requirements of nutrients, space etc even if you don't change nutrients. Diatoms have a growth rate that exceeds dinos, and over time, they will replace. Diatoms are more widely edible to snails crabs etc, and are less annoying to hobbyists.
Like I said, your system sounds mostly happy, and your dinos seem to be just an annoyance. That means your treatments should be very conservative and minor. I'd add Si, and a little PO4, and then make sure NO3 doesn't zero - but I really wouldn't move it much if my acros were happy and I'm just dealing with some brown sand.