Flourish Comprehensive isn't really so comprehensive. Take a look at a partial chemical analysis:
Total Nitrogen: 0.07%
Available Phosphate (P2O5): 0.01%
Soluble Potash (Potassium): 0.37%
Iron (Fe): 0.32%
Notice that the macro nutrients are only included in trace quantities, less than or barely more than iron!
If a nutrient is very near zero, that is in fact a form of algae control. A system called PMDD used to take advantage of that, by keeping phosphorus in the water as close to zero as possible. It's just not a good form of algae control - if the water content doesn't have enough nutrients to support algae, it's even worse for the plants. They're essentially forced to rely on the nutrient content of the substrate alone, which may or may not be adequate.
Add a trace of the limiting nutrient to the water, what happens? The algae can now grow freely, they require little. But this won't substantially impact the plants, which require more. If they were sluggish or suffering before, they will still be, and unable to exert their full anti-algae effects on the tank - which would more than counteract the nutrient availability to algae.
So even though dosing Flourish Comprehensive might have been a small step in the right direction, it can actually make things worse.
Now Flourish Nitrogen/Phosphorus/Potassium are a different story. Their primary ingredients, combined for all three:
Total Nitrogen (N): 1.5%
Total Phosphate (P2O5): 0.3%
Soluble Potassium (K2O): 5.8%
Huge difference between using these and Comprehensive. If you really do have a macro nutrient deficiency, it will take these to solve it. Or some other quality brand of liquid ferts. Or dry ferts.
As for the nutrient content of Aquasoil, I'm afraid I know far too little about it to offer any accurate or helpful info; better to leave that to someone else.
Ramshorns do indeed multiply, in response to the food available. When that food is algae, that's actually a very good thing, with their population tending to adapt to your need for a cleanup crew. Once the algae growth is made minimal by solving any underlying problems, the ramshorn population eventually becomes small too. Though personal preference weighs heavily here - what I might consider a small snail population and a few eggs, some others might consider intolerable. And I do like snails, just as long as they're not pond snails, which multiply far more on smaller food supplies. If you're not sure, skip them for now, as they'll be hard (or impossible) to remove later. Otos and the shrimp should be fine.