News flash, there are many subtypes of serotonin receptors. All have different effects, some are presynaptic, some are postsynaptic. That's not the only parameter that determines, their location in brain is important too. It's not like "Serotonin antagonists are good, agonists are evil". For example buspirone, a serotonin agonist, is used and can treat generalized anxiety disorder. Besides that, there're actual empirical proof, that SSRIs improves life quality of people with depression.
Your advice about cyproheptadine is both useless and potentially dangerous. It's mainly a histamine antagonist, one of the most powerful ones. Yeah it antagonizes many subtypes of serotonin and dopamine receptors, but antihistaminic activity dominates them. Thus it's sedative/hypnotic effects are far more pronounced, which renders it practically useless. And it can be dangerous for people with glaucoma, who are not aware of it, it can worsen it. Wellbutrin is often prescribed to counteract SSRIs side effects, but don't ever use it without consulting a professional.
Oh one last thing LSD is most definitely NOT a serotonin antagonist, it's a typical psychedelic, it's a 5ht2a agonist.
Ps:These don't come from someone who just read about these stuff. I have been on Prozac and have tried almost every single histamine antagonist(including cyproheptadine) on the market because of my allergies. Prozac transformed me into someone who I cannot believe that was me. And that guy was calm, focused and confident. He got me into med school. As soon as I stopped taking it I was my old self just like now, because none of my beliefs has been really changed, that's where the main challenge is.
Good luck on your journey