Any particular reason Homing Sliver isn't there? Being able to slivercycle is quite nice. You can search for overlord, manaweft or even crib swap (it doesn't specify creature, only sliver CARD).

I think it homogenizes the gameplay a bit, but can be much needed if you're locked into a position with no immediate answers.

Dormant Sliver is also pretty nice but you won't be able to attack, but if you get gemhide/manaweft out, you can start populating the board while drawing cards, then sacrifice them with Basal Sliver, Cautery Sliver (not idea since it costs ) or Necrotic Sliver. Might not be ideal with morophon as your commander as it's easier to tutor with Overlord.

Overall, classic sliver deck!

Oops, i was wrong on the ETB triggers above... DOH! I can't edit the commend to correct sadly.

I would very much recommend:

Garruk's Uprising as it gives you trample for the big bois, can cantrip AND gives you draw power on ETB. If you cast a hydra with > 3, you get to draw 2 cards, assuming Zaxara is on the field.

Kiora, Behemoth Beckoner Gives you ramp AND ETB triggers. Same as G's Uprising above.

Elemental Bond Have you noticed a pattern yet? More expensive than the others because of the Power 3 trigger vs 4; but still a really sick card to have.

Simic Ascendancy Alternate wincon and pretty easy to get twice since, if a hydra enters with 6 counters, you get 12 if you control Zaxara due to the token getting the +1/+1 counters.

Open Into Wonder Straight up ends the game if a creature is big or your board state went unchecked for too long. Won me plenty of games. If it can't end the game, you can still draw plenty of cards to play on main phase 2.

Finale of Revelation Essentially just an X draw but with additional benefits for going all in. Obviously, Zaxara is needed to get a 10/10, but it's way better than Even the Score; the extra 2 cards drawn is meh and you lift the 4 card restriction. Blue Sun's Zenith is technically better being at instant speed, but you get WAY more value in EDH out of Finale, especially with mana doublers/triplers.

Herald of Secret Streams A more $$ card, but makes your creature with +1/+1 unblockable; usually allows you to kill 1 or 2 players that turn and end the game if it goes unanswered, buying your other threats more time.

Curse of the Swine To exile high prio threats. Giving opponents 2/2 creatures doesn't matter. Gets around indestructible or creatures that are overly powerful such as voltron creatures. I would honestly swap this for Spinning Wheel Kick as that relies heavily on your commander; which Zaxara is already a high prio target.

Those are all the suggestions i have for budget; most of the others i'd make for budget are counter spells. My Zaxara deck should be on my profile if you're interested in higher $$ cards.

(I had something about mind grind, but i missed the section that explicity talks about it! DOH!)

Amazing deck; I'm bookmarking this one! Also, really awesome job with the description!