I don't really know if this helps, however I do see a few alterations you COULD make.
Ob Nixilis, the Hate Twisted --->
Underworld Dreams (not necessary unless you seem to not get to play Ob Nixilis often)
Remove
Sage's Row Denizen, as the primary focus of your deck is not to mill, however it could be a good sideboard card.
Remove
Vraska, Swarm's Eminence, as you don't get any ETB triggers from her and she doesn't really do much other than create mid-game blockers.
Adding
Cavalier of Gales would allow you to draw an immense amount of cards, but might draw you too many cards. You could also add
Lazav, the Multifarious to help you find cards you want with his surveil ability. These cards paired with
Thassa, Deep-Dwelling creates incredible amounts of value with the surveiling and card draw but might mill you out if you aren't careful.
Not saying this would be a wise choice, however it could be really fun. But you could make a deck around the Amass ability with cards such as
Gleaming Overseer,
Enter the God-Eternals,
Dreadhorde Invasion,
Eternal Skylord, and
Vizier of the Scorpion. All of them will buff your token as well as having your commander doubling the Amass triggers. When paired with double proliferate triggers as well as maybe adding in some Simic counter moving tricks, such as
Combine Guildmage, Repudiate//Replicate, and Hadana's Climb//Winged Temple of Orazca.
Roalesk, Apex Hybrid triggers twice when it enters from Yarok #VALUE (I never thought you could do a Voltron deck with a token until now haha.)
Simic Ascendancy also could work by putting pressure on your opponent with the Amass strategy.
You could also add Impervious Greatwurm as a massive creature to abuse and
Moss Viper as an early game blocker.
If I think of anything else I will be sure to let you know, sorry for getting carried away. I love deck building, and this seems like a really fun card to build around.
(Sorry if this didn't help!!!)