Muktol

[[Stuffy Doll]] isn't a legendary creature, so it can't be used as commander. Even if it would be, it also is a a colourless creature so you can't use cards with coloured mana-symbol of any kind in your deck.
You could tag your deck as "Highlander"-deck, as those use other building rules.

What about using [[Fastbond]] with [[Ramunap Excavator]]? When combining those with [[Retreat to Kazandu]], a simple [[Evolving Wilds]] could lead to nice combo. [[Retreat to Hagra]] could also be an option tough that is out of your colour combination at the moment.

As for the manlands/ artifacts, you could use [[Darksteel Brute]] as he's indestructible and [[Creeping Tar Pit]] would also fit into your colour-scheme and it's unblockable.
Another idea could be the use of vehicles...man your lands and let those crew the vehicles.

[[Executioner's Capsule]] instead of [[Easy Prey]]? Can serve as fodder for your [[Salvage Titan]] to help him come back into play.
[[Servo Schematic]] could also be an idea instead of [[Cogworker's Puzzleknot]]

Personally I would reduce the number of different cards you're using in favour of a more stable game-plan. Additional carddraw is a good thing, but relying on (in a deck that doesn't have carddraw as main strategy) can backfire very easily in my opinion.
Cards like [[Demystify]], that you don't use every game, should be replaced with more versatile (or potent) versions ([[Disenchant]], [[Echoing Calm]], [[Heliod's Intervention]], [[Leave No Trace]], [[Light of Hope]]) or moved into the sideboard.

Here are some additional suggestions if you want to replace cards in your deck:
[[Empyrean Eagle]], [[Jubilant Skybonder]], [[Sephara, Sky's Blade]], [[Thunderclap Wyver]],
[[Favorable Winds]], [[Rally of Wings]], [[Battle Screech]]

Just be aware that the 2 tokens spawned by [[Hero of Bladehold]] will not activate [[Hellrider]]'s ability.

With all the spells that also force you to discard, you could add some of the madness-cards that came with Eldritch Moon:
https://www.mtgvault.com/cards/search/?q=madness&searchtype=text&ps=20

There are so many ramp-options out there, that it's hard to keep track of. My favourite would be defender-ramp, using [[Axebane Guardian]], [[Overgrown Battlement]] and a bunch of cheap defenders like [[Order of the Stars]] or the classic ramp in the form of [[Harrow]] or [[Arbor Elf]] + [[Utopia Sprawl]]

A short deck- and how to play description wouldn't hurt anyway ;)
Personally I would use other ways to get [[Progenitus]] into play than [[Garruk, Caller of Beasts]] (Which is currently the only way you can get him into play, unless I've overlooked something). [[Elvish Piper]] would be the most prominent, but there are alternatives to it: [[Iwamori of the Open Fist]], and [[Dramatic Entrance]]. [[Fierce Empath]] can be used to get Progenitus into your hands

[[Kaya's Wrath]] could be an option (at least sideboard) against aggro/ swarming decks.

I've recently spoken to my friend about this deck, especially when I mentioned [[Conclave Mentor]] to him. He seemed very interested in that creature, and said that he wanted to add it to his deck. I'm really interested with what he will come up our the next gaming-night (which will unfortunately take some time).
Personally I would go for [[Conclave Mentor]] or [[Barkhide Troll]] as replacement for [[Enduring Scalelord]] and [[Seal Away]] as removal. I will sent him your suggestions, let's see what he does with them ;)

Don't know if your friend is still playing but: Ikoria brough [[Bastion of Remembrance]] (basically [[Blood Artist]] 4-8, but harder to remove) and [[Serrated Scorpion]], which might fit into here.
Personally I would also add something that lets you sacrifice your creatures and benefit from this. There are numerous options for this, but [[Bloodflow Connoisseur]] or [[Bloodthrone Vampire]] would be some of the cheaper ones. Some carddraw in the form of [[Village Rites]] might also be an idea.

War of the Sparks brought some good landfall/ proliferate cards that you could add here:
[[Evolution Sage]] (Works like a charm with [[Nissa, Who Shakes the World]]'s ultimate), [[Pollenbright Druid]], [[Contentious Plan]] and [[Courage in Crisis]] could be worth a look.

A short deck- and how-to-play description would be a good addition. If you don't know what to write, you might take a look at
https://www.mtgvault.com/muktol/decks/why-you-should-add-description/
[[Ball Lightning]]/ [[Groundbreaker]] come to my mind when talking about fling. [[Brion Stoutarm]] could also be an idea, tough he's out of your choice of colours. [[Elvish Mystic]] could be replaced with [[Utopia Sprawl]]. could also help you with mana-fixing.

Personally I would sideboard [[Naturalize]], normally you don't have to deal with artifacts/ enchantments in every match. [[Hydra's Growth]] sounds like overkill to me, especially when you have [[Assault Formation]] on the battlefield. A single [[Wall of Denial]] will need 3 turns to defeat an opponent, while [[Hover Barrier]] needs four, so [[Hydra's Growth]] will make nearly no difference.
I would also replace [[Rolling Stones]] with [[Arcades, the Strategist]] or [[High Alert]] as they not only allow your creatures to attack but also bring the toughness-bonus of [[Assault Formation]] to bear.

Further suggestions:
[[Axebane Guardian]]...not as tough, but can serve as mana-fixer. Tough you may want to add a mana-sink to use the mana produced with him. [[Overgrown Battlement]] would be another variant, quicker, variant tough it only produces green mana.
[[Order of the Stars]]... one of my favourite defenders, not real tough, but hard to deal with.
[[Stalwart Shield-Bearers]]...again, not really one of the toughest defenders, but gives all your other defenders +0/+2
[[Drift of Phantasms]]...can help you to get cmc3 mana cards in your hands.
[[Seaside Citadel]]...to solve mana-problems.
[[Indomitable Ancients]], [[Bar the Door]], [[Slagwurm Armor]], [[Wall of Reverence]], [[Wall of Shards]], [[Favorable Winds]]

A short deck- and how-to-play description would be a nice addition. If you don't know what to write, you might find some ideas here:
https://www.mtgvault.com/muktol/decks/why-you-should-add-description/
[[Ram Through]] instead of [[Prey Upon]]. Personally I would remove {{Fog]] and add something like [[Aspect of Hydra]] or similar cards.

I didn't see [[Wrangle]] as a way to untap things, but more as control-option. If you want something to untap other things. without the versatility of [[Wrangle]], then you could take a look at [[Magewright's Stone]] or [[Thornbite Staff]]. The Staff has a nice synergy with dying creatures, and if you throw in e.g. [[Skirk Prospector]] you can do an combo, but it's also more expensive to play (3 Mana Stone vs. 6 Mana Staff)

Personally I would trade [[Goblin Warchief]] for [[Goblin Chieftain]] as I think you will profit more from the +1/+1 than from the lower cmc. Also [[Goblin Tunneler]] (or [[Break Through the Line]], due to the haste-ability) instead of [[Sneaking Guide]].
It would be interesting how well [[Conspicuous Snoop]] really works. Could you write a short comment after you have done some testgames?

[[Signal Pest]], [[Larger Than Life]], ([[Wild Defiance]]) could be ideas, and I would lower the numbr of Forests by 1-2

Depends on how you want to achieve victory here. It looks like the beginning of a goblin-swarm deck, but with all those 2-off's it will be hard to get a continuous performance.
[[Conspicuous Snoop]] is a strong card, but it only works with [[Krenko, Mob Boss]] and [[Sneaking Guide]] and the chances of getting those atop of your library while you have this little fellow out are slim.
I would also try to get the deck modern-legal.