Wait, stay at that drawing board! This isn't a reason to abandon the strategy, since you have to be pretty unlucky to not be adequately boosted by the time the first poke faints. "Adequately boosted" needs to be defined as it relates to your purposes, though. What's the worst case scenario here? We're missing five boosts, of course, and what are the most important boosts that Toxicroak needs? I would say Speed is definitely most important, then either Defense or Special Defense. Split the difference and we're looking at no Speed boosts and only +4 Def and SpD.
Think about that for a second, though. And think about it optimistically, as well. What are we really looking at? First of all, I would recommend using Cross Chop or DynamicPunch over Drain Punch in Toxicroak, since you're going to be boosting your accuracy before attacking anyway because you have to set up. I also recommend 252 Atk Adamant and 196 Speed EVs, the rest in HP. With this, the the worst case scenario I think you're envisioning, we have a 692 Atk EV Toxicroak with two 100% accurate, STAB, 100+ base power moves with fantastic side effects, at +6 attack, tripled Defenses that they'll only be able to hit a third of the time thanks to +6 Evasion.
But that's not all. The foe poke isn't going to magically disappear after 16 turns of Protect. You're not tricking it, so you don't even have to worry about it running out of, like, Stone Edge PP or something and Struggling off your screen. You're going to still be able to Accupressure if you have to, which will only be the case if you don't ever boost Speed in 16 turns. The chance of not boosting Speed once in 16 turns is a mere 0.7519% chance. I like my chances there, especially considering that when I Acupressure next, that chance is merely 0.3%, and then 0.0752% on my 18th Acupressure. That's having to risk two turns with super Evasion and either +4 or +6 Def/SpD (one will be boosted on turn 17 and the other on turn 18) and probably a Sash.
Turn 19 of course you'd be guaranteed to boost Speed if it actually came down to that, and then would be fully set up. With 196 Speed EVs you're guaranteed to outspeed everything in the Subway after one sharp boost which is why I've suggested that EV Spread...it's not like you really need Defenses on your Protect sweeper on a TruAnt team. But do you see how extreme this is? 0.75% chance you don't boost Speed once in 16 turns is astoundingly low, and with the safety net that all your other boosts you're really not in any real danger at all.
As I said for my DRAPULA back in the days where we couldn't Acupressure behind a Sub, I only needed about 12 sharp boosts: one in Speed, one in Accuracy, and basically two each in Attack, Defense, SpD and Evasion, with two in Attack to make up for not having all my Defensive boosts. The main issue, of course, is paying attention to all these boosts and the amount of time they take anyway, especially with having to Protect every other turn as well. I haven't used Drapion in a serious capacity when I realized it wasn't as foolproof as it was in DP even with the Sub-Ace nerf gone, since stuff like Scrafty, WW Skarmory, and the Musketeers were still pains even with a setup Drapion. Add to that the 20+ turns I needed to get set up (with Subs in there) and it just very literally was and is not worth it when there are "Bullet Train" teams like Carl's (his term) and Peterko's and mine that can win in 4-6 turns a good out of three times and still be incredibly reliable about 30% of the balance of the battles. Time remains the great equalizer in the Subway, and only because the Subway2 is identical to the Subway would I even entertain the though of giving Drapion another shot (with Dragonite because they have great synergy as I may have mentioned before).