How Should I Go About Getting 100% Critical Rate?

I agree here, except PDR is still quite valuable.

I know you asked for how to get 100% crit rate. However, even with AB's, I don't think additional crit rate is that good of a stat compared to the good weapon potential lines (%atk, % total damage, % boss percent, sometimes pdr).

My advice;
-Never sacrifice a weapon potential line for crit, unless you get it in the process of two other good lines (ie, don't shoot primarily for crit rate on any weapon).
-If you can afford boss nebs, don't go for 100% crit, otherwise, use the neb/MM card combo.
-Also, if you plan on doing a lot of chaos root, PDR nebs might possibly be better depending on how much ignore you already have. At that point though, I would assume you would be close to being able to afford boss/higher pdr nebs, in which case you should still never sacrifice a weapon potential line, bonus pot line, or neb slot on crit.

I'm currently sitting at ~75% PDR (minus Ambition because who knows if that works), and my new weapon has yet to be cubed. Current weapon has 15% ATT/6% Total Damage.

Boss Nebulites are way out of my price range and I really doubt I'll be able to do CRA any time soon, so I think I'll go with the Nebulite/Marksman Card combo. Thank you guys for your opinion!