Sub-panel wiring, grounding, disconnect
There are so many mistakes here that I'm very concerned about the fact-finding process which led to them (or lack thereof).  Because "writing a book" is not the SE format here, I'll be lightly touching on each.   

But speaking of "books", and going back to that fact-finding process, I'd strongly advise hitting the library for some reasonably current books on electrical, and read them through - the goal being to get a well-rounded primer on the subject.  Google gives you swiss-cheese knowledge because it only asks questions, and without a primer you don't know which questions to ask.  Home-improvement store clerks know worse than nothing.

Anyway on to it.  



Voltage drop - the first question on a 180' run.  Here I deviate from stock advice, which is keep rigidly &lt;3% of breaker trip.  Actually, 3% isn't in Code anywhere, and it's a very coarse rule of thumb - 5% isn't wrong.  And you should be computing voltage drop based on practical loads, i.e. what you expect to be actually pulling on a regular basis. 

Aluminum wire - you didn't mention aluminum but you should be using it for large feeder >=4AWG.  That means a size bump from copper.  I often see people take dangerous shortcuts to save $30 on a project only to spend $800 more than they needed to because they're afraid of aluminum. Aluminum didn't work for small wires like #12 using alloys made for high tension lines.  Feeders were always reliable in that alloy, and they've improved the alloy too. 

The service panel.  Everybody does it the first time.  First get a huge panel - when you're buying a panel, spaces are cheap.  Regrets, however, are expensive - and believe me when the power is there, you'll want to use it. 240V heaters, on-demand hot water (30A/240V is fine), electric car, prosumer shop tools that are 240V, these things chomp up breaker spaces 2 at a time. A 30-space breaker is only 15 loads.  A 42-space is not uncalled for. 

Honestly a 70A subpanel that's only 2 spaces makes no sense as a product.  It could only serve as a shutoff switch for one single 30-70A machine.  It is wholly inappropriate to power a variety of circuits, as it sounds like you want in this shop. 

Second, you need a "main breaker" because that's your shutoff switch/disconnect (5). The most economical way to buy a disconnect is simply select a panel with a main breaker.  Amp rating? Nobody cares, it's a disconnect.  So feel free to get a subpanel with a 150/200A main breaker if that'll give you a sensible number of spaces. 

For aesthetic reasons, I strongly prefer panels with an obvious, separate main breaker/disconnect, not one where the main is in the rows of regular breakers and only found with a "main" label. I want a visitor to be able to find the shutoff in a hurry, in half-light, in smoke.  

Separate Ground bar - correct.  Neutral-ground isolated - correct. 

Ground rod (1) - unless you've done the arcane wizard measurement to affirm &lt;25 ohms on the ground rod, you need two rods some distance (10'?) apart, unless your local code says otherwise.  Code often has local amendments based on local conditions.  

Ground wire - the only conflicting information there is "All the Codes say this is required" conflicting with "I don't want (you) to pay for it" (because I want to quote you a better price than my competitor, who is following the law).  You need a wired ground wire back to the house.  Period.  Dirt doesn't conduct electricity well enough to serve as a substitute, if it did, we would just put THHN insulation jackets on dirt. 

Ground size bump vs. voltage-drop size bump (3).  When you make wires bigger for voltage drop, you must make the ground wire bigger in proportion to cross-section. That math sounds hard, until you realize wire gauge already works that way.  So if you do a +2 AWG size bump for voltage drop, simply do a +2 AWG size bump for the ground also.

THHN wires inside conduit (2) - correct. You use THWN-2 wires inside conduit. (almost all THHN wire is dual-rated THWN-2, so people don't bother distinguishing them). 

Don't push the limits of conduit fill. (4)  Those limits are intended for pro electricians with experience, special pulling hardware, lube, and a winch on their truck.  As a novice doing your own pull, be generous with conduit size, or else you could find yourself boxed into a corner and having to bring out an electrician to finish.  In this economy electricians are not hungry, and won't return your calls unless you give them most of the job (and let them bill for that). Also if there are any defects in your work so far, they won't touch the job with a 10 foot pole unless you hire them to fix it all. So pencil in $1000+ for that. 

No need for  GFCIs downline of GFCIs (6) unless you want to play a "Yo Dawg" joke on yourself.  There's no penalty for doing so; it's just wasteful.