I'm not sure if it's too 'non-Dragon', but I've had good results fiddling with Orcish Lumberjack, especially in a deck with Mwonvuli Acid-Moss, Lumberjack and Acid-Moss gives you access to a true Sinkhole effect (you can even play it on turn 2), and it can technically find a non-Basic Forest fwiw. Technically you only need to tap the Forest and sac it to Lumberjack to play an Acid-Moss, but Lumberjack can also give you a turn 2 Dragon that costs 5 mana, does your meta feature a lot of cheap kill spells? If not I wonder if that'd be worth your while? I don't know for sure if Atarka had Orcish servants, but Orcs live on that plane, so the lumberjack has a tangential connection.

Also, since it's gone down a ton in price, what about Utopia Sprawl? It makes your Forests better, and you can't cast it without a Forest I understand?

Since you're actually in Gruul, I would just run Mountain Valley over Evolving Wilds, this can find a Dual, or if you're desperate it can still dig out a Basic (that you can use the same turn Evolving Wilds would allow you), the only issue is if decks play cards that blow up non-Basics in your meta (most don't).

It's not entirely budget, but Up the Beanstalk is a very strong card if you actually plan to cast those big Dragons, and it replaces itself.

I'm not sure what your meta is like, but if people run Stax stuff that can ruin your day, you can include Dire-Strain Rampage, this sorcery offers you a Harrow effect if you wish, but it can Naturalize something, or Ghost Quarter someone's important land in a pinch. The best part is that it has Flashback. It's not the fastest effect, but I feel like it's certainly good enough for sideboard (especially on a budget), and since it can also just ramp you it might be fine mainboard, ymmv.

It's an ETB tapped land, but Treetop Village is a reasonable attacker if you end up desperate, with your curve it seems plausible that you'd be able to play an ETB tapped land, maybe x2?

Do you mean cheap 'fetch' options like Evolving Wilds, or pricey stuff like Verdant Catacombs? Proper Fetchland are really strong cards, and very few things screw you over for playing them. People typically benefit from Fetches if they have landfall effects, usually you need quite a few, or very good ones, or they are a lot of money for relatively little value, especially 2 colour deck. This changes a bit if you've got other synergies, especially cards that let you play lands from your graveyard, Fetch lands are incredible in decks like Muldrotha, the Gravetide, where getting a single one in your graveyard technically means you'll never miss a land drop (unless something happens). In your colours people often run Fetch lands with cards like Crucible of Worlds and Ramunap Excavator (there are more options, these are just very popular ones that synergize with effects that give you the ability to play extra lands, such as Burgeoning). The nicest thing about these sorts of combos is you can also throw in something like Strip Mine/Wasteland to generate a great deal of advantage over time.

Looking at your deck for a bit, it doesn't have a lot of 'bad' cards kicking around, so it's hard to improve your deck without raising it's power level. Arguably fetches don't increase your power level, they just make your deck a lot more consistent, but with enough synergies you can possibly shave a few lands if you ran all of the true Fetchlands that can find a Swamp or Forest (at this point there is a range of dual lands that have the Swamp/Forest typing, but most ETB tapped fwiw. I'd run at least 3 duals you can fetch out if you're going to run a bunch of Fetchlands, as well as a smattering of Basics.

Hope some of this helps!

I would take a look at Open the Way, if it doesn't get reprinted in the near future the card is going to get bonkers I suspect, the floor of 3 mana for your next land is both playable and less hassles than a true fetch effect (you don't have to shuffle for it), but if you play it mid game for 6 it's a bit unreal.

Have you ever thought about Mask of Griselbrand? Flying and Lifelink are very good keywords, and this can be a VERY effective form of protection for your Commander, people can't let it die or you get a new hand for almost nothing.

Since people have to attack with everything (and can't send them at you), Rite of the Raging Storm might do some work for you, if an opponent can't deal with 5/1 attackers they are very easily killed.

Not sure how big your Thantis gets, but I like Mandate of Abaddon in my Kresh deck, Kresh can get big enough that this is not just a 4 mana one sided wipe, it also means I'm going to Commander damage someone out of the game.

Not sure how often your Commander gets locked down with stuff like Song of the Dryads, but Angel's Trumpet can be a decent back up.

It's a bit of a weird niche card, but I love Tempting Licid in a Goad deck, this guy can be attached to a creature that you forced to attack at instant speed, and now the opponent that creature is attacking is getting blocked by the kitchen sink, and as soon as those blockers are declared you just detach the licid and go on your merry way. I think most people find the card too confusing to play, but the effect is extremely strong, useful to threaten attacking or blocking creatures. It can also be a one player wipe if your Commander is big enough.

Thanks for the upvote for my Meren deck, it's a deck that is near and dear to my heart, and Chatterfang is a worthy addition.

I'm glad a few of my idea were worth trying out in your SQUIRREL!!! deck, I hope they work out!

Have you ever tried Revel in Riches? It's not a bad card with your Commander, especially since you can use treasures and squirrels to kill small stuff, very handy vs decks with plenty of X/1s. I was also curious if you have tried Mitotic Slime, it's an impressive source of bodies/tokens for a Golgari deck, it's very possible to build combos with it and Demon of Dark Schemes, and the Demon also kinda likes Chatterfang since he makes bodies while other cards make treasures!

I was so interested in this deck that I finally pulled out Haakon from my Meren deck, replacing him with Chatterfang. This deck convinced me that he's probably a better fit in a combo oriented deck.

Thanks for commenting on my Sunastian deck, that deck is a role play deck, and as such I've put up some lore to explain it. I actually have 5 'related' decks, a pod of Legends decks that each have lore around the Commander that is entirely made up, but tbh if those folks didn't have something else going on how did they end up Legendary? At least Sunastian can tap for Sol Ring mana, poor Tobias Andrion is literally a 4/4 for 5, he's a vanilla that fails the vanilla test.