The effects of propanolol are exacerbated by other meds, like Paxil (paroxetine).
Back when I was on Paxil (40mg a day), my neurologist prescribed me Propanolol to try and deal with my migraines. She recommended I take 200 mg a day. After just two or three days of taking both, my heart rate dropped to below 40 bpm when I was resting (I'm usually at 80-90), and my blood pressure dropped as low as 50/20. (I usually have a perfect blood pressure of 120/80).
It's worth considering. I still have a full box somewhere that I was hoping to OD on with a cocktail of other stuff, but I seem to have lost it unfortunately.
Edit: It is likely that I naturally had a sensitivity to Propalonol that made it worse for me, but medical literature suggests they should never be taken together.
Edit 2: Propalonol on its own might not be enough though, unless you have a crazy amount of pill.
A quick search suggest that there are successful suicides with Propalonol as the drug of choice, but there are also failures. Someone took over 5 grams and failed their attempts.
My guess is that you'd either need way more than that, or take other meds that will make Propalonol more potent. I'm sure a quick search could let you know what drugs interact severely with Propalonol and augment its effects.