On first glance, I'd say you need more ramp. Your commander is rather pricey, so you want to have your mana be reliable to cast her. I'd also add at least two more lands, but I guess that's preference.
Other than that, I think the deck needs some redundancy in terms of forced combat. From what I saw, Thantis is your only way to force combat. Maybe add some cards in the 99, so you don't have to completely rely on your commander.
Druids' Repository is awesome, I've never even seen that.
I'm unsure about the Mega Chonkers section, whether or not it synergizes strongly enough with Thantis.
I'ma make my Thantis WIP public - maybe you'll find some ideas in there. The way it is at the moment, I'll still take out ~44 cards and add ~10 more lands and then check what I want to swap in from the Maybeboard.