Okay so there's a lot to unpack here. One at a time:

• The math on Giantslayer vs Warmaster is essentially, if you average about 2.5 attacks per turn, Giantslayer wins out, otherwise Warmaster wins out. So, specifically on Baerdall, he'll get three attacks on the first two turns, and then either two or three attacks on the third turn, depending on whether he uses his A1 or his A3. Since you can time that one perfectly, as it's a 3-turn cooldown, you can assume he averages 3 attacks per turn, and thus Giantslayer wins out, for him.
• Next, specifically comparing Baerdall vs Kael, ignoring the decrease DEF part - no, he will not outdamage Kael. Despite the extra damage from Giantslayer, and the fact that he has slightly higher multipliers on his abilities and slightly higher base ATK, the damage from poison will almost certainly outperform the damage from attacks. You need truly end-game level gear for attack damage to outdo poison damage.
• However, if you are currently NOT running a decrease DEF champ, you have to consider the value-add of how much extra damage the *rest* of your team gets from the decrease DEF. The math there is considerably harder. You didn't mention which level you're doing, but given you're still using Kael, I'm going to assume Brutal. Brutal CB has only 598 DEF, which means the 60% decrease DEF brings it down to 240 DEF. That means going from about 28% damage reduction to about 10% damage reduction. That's a *very* significant damage increase. I'd have to know the exact amount of damage the rest of your champs do, but my guess is that if you have no other way of getting decrease DEF, Baerdall will be worth running instead of Kael.
• In terms of Toxic sets - the math here again is complicated. The problem is that Baerdall will be hitting multiple times - as indicated before, 3 times per turn. Toxic has a 75% chance to place a 2-turn debuff, which means before the first debuff drops, he'll get nine attacks, for an average of 6-7 poisons. That seems great, on the surface - the problem is that if you are reliant on other debuffs, there is a very good chance he'll end up blocking them by taking up too many debuff slots. Typically you want to run DEF down, Weaken, and ATK down, which is three debuff slots. On average, you'll be fine. But you could theoretically land up to nine debuffs, which means you may end up blocking one of those others. Still - especially if he'll be the only debuffer on your team, Toxic is probably a good bet, yeah.
• Lastly, in terms of being Void - largely speaking that's a wash. Yes, you avoid affinity problems - but you also lose out on affinity bonuses. I don't consider him being Void to either be a pro or a con. It's just a thing.