You don't need to take metoclopramide across 3 days, whoever said that doesnt know what he's saying or just being extra careful, 30mg metoclopramide is good enough, metoclopramide is moderately lipopholic (meaning it passes the brain blood barrier at a decent rate) it has high oral bioavailability (80%), It's water soluble so it gets absorbed in the GI tract relatively fast, it survives first-pass metabolism at a decent rate, peak plasma levels usually occur after 1-2 hours posts ingestion (this is when metoclopramide's effect is at it's strongest) it takes 5-6 for metoclopramide to be cleared from the body, once you ingest 30mg, metoclopramide it'll be active for like 3-4 hours after 1 hour of ingestion
You can take 30mg of metoclopramide 90 minutes prior to ingesting anything lethal, if you're paranoid and you want to be sure you will never vomit you can try to full emesis suppression through aggressive pharmacology (Metoclopramide for D2, Ondansetron for 5-HT, Aprepitant or rolapitant for NK1, Diphenhydramine or scopolamine for H1/M1) but this is overkill and unnecessary and just complicates things
If you're going to overdose on something problematic that takes time to be digested like hundreds of pills of something with low water solubility then it becomes justifiable to take 10mg of metoclopramide every 3 hours but you'll probably also need ondansetron or full emesis suppression in this scenario
As for LSD, personally i wouldn't recommend psychedelics, but if you can get your hands on opioids or powerful benzodiazepines like diazepam and xanax that would be ideal, also, keep in mind pentobarbital is still a barbiturate so naturally you would feel relaxed and calm once it's in effect, just not euphoric in the way opioids make you feel, so lsd and benzodiazepines might be useless if you're not seeking euphoria