The beginning is a little odd to me, as seeing bugs/animals is more like what I've heard people get from diphenhydramine. That's the other common OTC cough syrup ingredient, (aside from tylenol, obvs) so it's possible you mixed them up. It's also possible that's just how you are on DXM, though. It's a dissociative which can pretty much be a deliriant at high dose, and shadowy critters are common for that class.

Generally speaking, as long as you're not taking so much you get serotonin storm (you have to go WAY past even a recreational dose for that unless you're on something else serotonergic) or doing it so often that it messes with your liver function, DXM is scary way before it's dangerous. It can be, however, quite scary. Even at lower recreational doses, what's referred to as first and second plateau experiences, DXM really likes to convince you you're dying, even though absolutely nothing permanent is happening unless you try to use stairs or something stupid like that. Once you get past that paranoia, it can be really fun, but it might take practice. You might wind up in a weird spot where you think you're dying/dead but you don't actually feel scared of it. It helps to have somebody to supervise you so they can take your phone away when you try to call 911 just in case for the third time in 20 minutes. I promise you won't actually die of it. The LD50 of DXM is stupid high, you're virtually guaranteed to puke or pass out first.

If you have the wrong set and setting in the third plateau, you can wind up in really irritating and confusing time loops in simplified versions of your real space that you can't seem to leave. (For a good experience, I find the best method is to put on some calming music and a Mandelbrot zoom before you forget how to use your computer, and to focus on that for as long as you can tolerate sitting upright.) Fourth plateau experiences are kind of harrowing even when you've been through it a few times. You might "find divine secrets," or you might just be scared and locked in a body that can't hear or see for a couple hours, or you might simply blackout and waste your time completely. As somebody who has had legit positively transformative experiences with the fourth plateau, I can't recommend it at all because you barely have any control over how it hits you no matter how many times you do it. I've never had access to any of the "real" psychedelics, but I am given to understand you can get to the right place much more reliably with any of those, and they won't leave you stuck wherever they put you for three eternity-hours.

Nobody at the ER is gonna yell at you for using drugs, especially not relatively harmless OTC ones, unless they think you were trying to commit suicide, which is pretty unlikely with DXM. That's your primary care physician's job; you can talk to them until you're blue in the face about how Olney's lesions aren't real and the paper about them was retracted years ago, and they'll still frown and tell you to get help with your lack of a problem.

If you ever decide to try it again, google something like "dxm plateau mg/kg." If I remember correctly, one of the first sources you'll find from those terms is a dipshit fed PSA about how DXM abuse is such a horrible problem, but they actually have the right numbers, which you'll be able to corroborate pretty closely from other sources. Plug in your mass in kg, and try to shoot for the middle of the third plateau. You can play around with your dose from there as you get more practice. Or you can just not do it at all, which I recommend, even though I enjoyed it quite a bit for several years. It can be fun, it can give you transformative insight, and it can also be a completely unnecessary and fake traumatic memory. If you get the dose wrong or you're not in the right place, the only thing you'll learn is that you should find another hobby unless you're willing to be quite disciplined about it.