Learning all of the stuff is going to take some time, but I can give you a few places to start.
Icy Veins: Beginners guide to seasons
A general overview of the current season from Blizzard
The season journey, Which is a sort of sidequest to progress alongside greater rifts. And the reward you gain from completeing the chapters in it.
There is also some new stuff introduced to the game while you were gone, somethings to look out for are Legedary Gems and Kanai's Cube, both of which are used for the augments you mentioned.
That should get you started, most of the old stuff is still the same, set bonuses are a whole lot more usable (required) now.
Other than all that, Welcome back and hope you enjoy the game 
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Diablo 3 is the best-selling Blizzard game of all time. If you think a 3 year content drought while Blizzard hemorrhages talented developers isn't a major disappointment, then fuck help you because nobody else can.
"Out of their own pockets" LOL
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Well I mention even more gold and gems so that you can get around to upgrading all of your socketed gems to Flawless Royals. Roll sockets into your chest and pants, and get 5x Flawless Royal rubies into your gear- that's 1400 more strength you stand to gain from that alone. That could certainly give you the edge you need to complete some solo GRifts, and yeah -as Angry_Roleplayer mentioned- you'll start gaining the Legendary gems that you can put into your Neck and Rings from those GRift bosses.
This is far from min-maxing, as it's just not worth crunching numbers at this point, but some pointers:
-Re-roll Armour in your necklace to Critical Hit Damage
-Re-roll Life Per Hit on your Pandemonium Loop ring to Critical Hit Damage
-Re-roll Life Per Hit on your Gloves to Critical Hit Damage
-As a general rule of thumb, you want to keep your Critical Chance and your Critical Hit Damage at a 1:10 ratio. That is, if your Crit Chance is 30%, aim for 300% Crit Hit Damage bonus. 35% : 350%, and so on.
-I know I told you to roll off a bunch of Recovery, but at the point you should be playing at, a great offense will make far more difference than having a balanced Defense.
-Make sure that in your Paragon allocation you're supplementing your Movement speed up to 25% (confirm this on your character sheet) as that's sort of the max run speed (there are a few exceptions) - anything left over should go into Strength for the time being.
-Barbarian might not be your forte, and that's ok! Personally, I couldn't get into the groove of Barb at first, so I tried most of the other classes until one stuck with me, or I just happened to get a string of good luck on gear. Remember, in general, the game kind of decides what your build is going to be. Sure, you may desire and build for a certain play style, but you might keep getting gear for an entirely different one - that's just the randomness of loot. The thing is, the entire game gets a lot easier once you have a "main" character that you love playing, and can amass gear, gold, materials for crafting, etc. You might try Monk or Wizard, and the better you get with them, the sooner you can re-visit Barb with a fresh view and more resources at your disposal.
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Seems to me like you just need to play in more public games. Join some Torment 1 Rift groups, grind out Rifts until you've amassed more gems, gold, and Greater Rift Keys. Then grind out Greater Rifts- you're practically guaranteed a Legendary or two per Greater Rift you take down. Progression in D3 can be a bit slow at first if you've simply come upon a rash of lousy luck, but if you're persistent, your luck will likely change and you can start forming goals based on what drops are in your fate.
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