5B > 2B > GH is really unsafe, don't go for that.
5C > 2C > 2D Is good, but can be escaped via instant block. 2C > 5C > 2D is tighter.
A really basic block string would be something like (2A) > 5B > 2B > 2C > 5C/3C > 2D. Strings that end in Deadspike are also nice thanks to the +3 advantage on block, but risky to go into in many situations, and require you to condition your opponent to not expect Deadspike.
6D is also a nice pressure reset tool, but like Deadspike, can be risky to use if you're too predictable with it. Most normals are cancelable into it, but it doesn't have great range.
We actually have a thread on frame traps, and that can be found here.
As far as mix-up goes, Ragna is really average in that respect, and requires a lot of conditioning of your opponent. You're usually better off with stagger strings and frame traps, and even just out-spacing them at neutral to get most of his hits. His 6B overhead however can be done off 2A, 2B, and 2C, as far as I know. Gauntlet Hades can be done off any special cancelable normal (meaning anything but 2D basically, but is just as slow as 6B (and not as rewarding, but that's cause it's easier to mix-up with).
Basically, learn your gatlings and build your blockstrings on the fly according to the situation you have your opponent in, as well as what character you're fighting, as everyone has different answers to Ragna's pressure. Don't be afraid to use meter to get that surprise overhead through rapid cancels, Ragna's heat gain is really good.