What's the current Ignore Defense Cap? Also is this good for Zero?

Depends greatly on what your current stats are. I would need to know how much PDRignore you currently have, how much %Boss/DMG you have, and what bosses you intend to fight regularly.

 rambling about details

2*30%PDRignore =/= 60%PDRignore, which I'm sure (including you) would know by now. In essence there is no true cap to PDRignore, unless you are one of the few classes that have a 4th job skill that ignores 100% of a boss's PDR. This is why I always introduce the term "PDR reduction factor (PRF)", which gives a much better idea of what's going on. If you have a 30%PDRignore line, then PRF for that line is 0.7, the smaller the better. Multiply all the individual PRF sources together and then multiply a boss's PDR to it, and that's all the PDR the boss will be left with. It's basically taking the math forward one step to understand it more accurately. So for just Lapis and Lazuli, the combined PRF is 0.7*0.7 = 0.49 i.e. these 2 will halve the PDR of whatever boss you're fighting. Is that alot? Yes... only if your PRF before Lapis and Lazuli isn't so small that any smaller doesn't make a difference (like PRFall = 0.1 and newPRFall = 0.05 on a boss with only 40%PDR is hardly going to make a difference at all, like 2% increase in DPS).

Hadriel

For my Zero, I got 91% Boss Att / 169% PDR for Alpha, and 141% Boss Att / 119% PDR for Beta. Yeah Boss Att and PDR is different for both of them, Beta gets 50% additional Boss Att, while Alpha gets 50% additional PDR.

My intended bosses to fight regularly are Gollux/Commerci/Hilla/Showa. First 2 are for loot, second 2 are for Elixir farming. May also do the lesser bosses if I'm really bored. Haven't done a real attempt at Arkanium yet. Unsure if I 'll even touch Magnus, even on Easy mode.