Adhering to the expressed conditions in the OP, I created a new account (while the 4.3 update was live) to see how healthy I could build it early on.
Got through the Sheggra battle to reach Hero Level 12, and was fortunate enough to acquire a treasure map via a seven times cascade.
Triple-vaulted that particular map, earning gems and glory in the process. I reinvested that glory into more maps, and from there, the cycle/process began.
Essentially, at level 12, without guild assistance, entirely free to play, I was able to accomplish the following with the resources earned via Treasure Hunt:
- Max-leveled the five kingdoms available at level 12 (Broken Spire, Adana, Zhul’Kari, Karakoth and Whitehelm - available with the 4.3 update) to ten, earning the +1 skill bonus associated with each.
- Cached just over one million souls.
- Acquired 700+ unique troops, including 100+ Legendary troops such as Yao Guai, Xenith, Wrath, Webspinner, Voidcaller, Umberwolf, The Great Maw, The Ghost Queen, The Dragon Soul, Tesla, Taloca, Queen Ysabelle, Queen Titania, Qilin, Sylvanimora, Sir Quentin Hadley, Orrery, NUTCRKR 1225, Kraken, King Oberron, King Highforge, King Bloodhammer, King Avelorn, Hyndla Frostcrown, Gorgotha, Glaycion, Essencia, Emperor Khorvash, Divine Ishbaala, Czernobog, Carmina, Captain Macaw and Basilisk, and many Epics such as “The Ironbeards,” Elwyn, Marilith, Mercy, Ragnagord, Rowanne and Zephyros.
- Fully traited and max-leveled (to their base rarity) forty-four troops, including Dwarven Gate, Egg Thief, Fire Bomb, Herdmaster, Leprechaun, Siren, Soothsayer, Sunbird and Valkyrie, with enough traitstones to be able to fully trait one Legendary (or an Epic plus, in some cases, an Ultra-Rare/Rare that uses the same traitstone) on roster that requires either Beast, Blade, Blood, Dark, Forest, Lava, Light, Mountain, Skull, Storm, Swamp or Venom traitstones. Plus, a few more Ultra-Rares and Rares, and quite a few Commons.
- Purchased both 500 gem armors.
Probably could mention this, I suppose.
And, probably these, as well:
and RNG trolled me with a second one for good measure:
And, if I include the handful of Orbs of Wisdom (via Orbs of Chaos received from consecutive logins at month’s end) with the arcane traitstones currently cached, I am able to fully trait each of the mythic troops acquired.
(Though, with the most recent update(s), I would need to reach Hero Level 18 before being able to apply traits to troops. 
)
Additionally, as of this edit (3/3/23), this account has 4,000+ gems and 1000+ gem keys currently cached. Current progress for a Hero Level 12 account..
All this by simply logging in each day, collecting daily login rewards, and playing one (mini) game mode.
Oh, and I would link this particular account to the GOWDB, but apparently, one must reach Hero Level 20 to utilize the “My Collection” portion of the site. And, I probably will never reach that level at this rate. 
Again, just another example of how healthy one can build an account solely through one game mode (Treasure Hunt). Certainly not a viable means of enjoying the full features of Gems of War, but an oft ignored method of bolstering an account.
And, actually, if you’re good at Treasure Hunt (say, you are able to create a vault every two to three minutes), it really is a consistent and efficient way to earn the four major resources early on in the game (gems, glory, gold, souls), while accumulating gem and glory keys in the process… by yourself.
Just another example of what silly people do, sometimes. 
