[EDH / Commander] Questions about some Jalira commander "staples"

[KittenSquared]

So I've been playing with the Jalira deck below and I've got questions about some cards I've seen used in a lot of other builds.

Mass Polymorph - I had this in previously and every time I drew it, I already had a pretty sizeable cluster of big guys on the field. It would be one thing if it just put them in the graveyard where I could eventually get them back with a rewind spell, but it exiles them. Am I starting my polymorphs too early or is this just not as useful as it initially seemed?

Whelming Wave - So much stuff in this deck isn't a Kraken, Leviathan, etc, and I'm not really keen on returning a 7 or 8 cost creature to my hand that I initially played for . Everybody else can likely replay the core of what they lost, but depending on how early it is, I might not be able to play it for a few turns, if ever (girlfriend plays a Nath of the Gilt-Leaf deck, so the threat of losing my big guys before I can play them is pretty real). Can anyone convince me its worth having in here?

Mirage Mirror - This one I actually understand, turn it into your/enemy's OP things temporarily to double up on effects and "steal" things without really stealing them. I just don't know what to drop for it.

I'm also trying to stay close to 100$ for overall price to stick with a group-wide budget suggestion, so please keep any improvement suggestions cheap!

Deck: Jalira Polymorph

[Morganator 2.0]

Mass Polymorph could be for two reasons. #1: flavor. Jalira is a polymorphist, so a card with polymorph in it sounds good. It's the same reason that Narset, Enlightened Master has Narset Transcendent. #2: other decks could benefit from this by being very token based, and then turning all of them into huge creatures (Instead of incrementally building up to it).

Whelming Wave is used because the majority of the big creatures in blue are Krakens and Leviathans. Inkwell Leviathan, Stormtide Leviathan, Scourge of Fleets, Lorthos, the Tidemaker, and Deep-Sea Kraken are all examples. You run most of these.