One weakness the deck may have is zero spot removal spells
Its slow and costs three colors, which are significant downsides, but Muldrotha could be good in the right deck
But there's another powerful midrange option that I love more than all those: Inti, Seneschal of the Sun
Actually, I want to write that Rinoa herself is quite strong as a commander is something I want to write
These are mostly black designs, considering the selfish nature of the symbiote carriers, and as a mechanical tie-in, some symbiotes allow you to exile creatures from your graveyard and turn them into +1/+1 counters, similar to how scavenge works
Angel of the Ruins Plainscycling can be useful in a pinch, and Angel of the Ruins removal can definitely work well, the card has a very high Mana Value and is an awkward fit here