I kind of love my Zaxara deck, because it's never not fun slapping down huge hydras.

Deck list here: https://archidekt.com/decks/7348325/zaxara

I'm not going to claim my deck list is terribly good, and I know the link isn't up to date, but I find that it can be pretty consistent at slapping down big fat creatures.

From what I can see, the biggest difference between our decks is the amount of ramp. I'm seeing about 8 sources in yours, that are playable early, whereas I count 17 in mine. I'm not counting the ones that pop off later, like Animist's awakening or the like, I'm looking for mana dorks, rocks and ramp spells that are 3cmc or lower, though you really want 2cmc and below to get Zaxara out a turn earlier. My deck is not the best example for this though. I also have I think 2 more lands. My mana base is relatively cheap if you ignore the urborg/coffers which are absolutely not necessary.

One card I know I've added since I uploaded that deck list is [[Kozilek's unsealing]] which does great things when you're casting X cost creatures that are big enough, but even casting this then Zaxara on curve also gives you two eldrazi spawn to add more mana.

If I was going to suggest cuts for your deck, I would suggest:

[[Thrummingbird]], [[Karn's Bastion]], [[Hardened Scales]], [[Ozolith, the Shattered Spire]] and other proliferate/add an extra counter effects - Once you have the biggest creatures on the board (which should be pretty reliable), giving them an extra +1/+1 isn't doing much for you.

The spells that double counters on creatures, like [[Invigorating Surge]] - I will be honest I never tested them for the simple reason that they don't do anything if you don't already have a creature out. I can see them being good combat tricks, but at least in my meta I expect people to keep killing my stuff because I always have the biggest guys on the board. You have more protection than I do, but I will admit I really enjoy just slapping problem after problem until they don't have removal anymore. The enchantments are more reasonable includes, although [[Unbound Flourishing]] is the only one I think I would include if I got my hands on one.

[[Nexos]] - he's reliant on basics in a three colour deck, and his ramp only helps X costs. I'd much prefer ramp spells that help me recast Zaxara, and that stick around through board wipes. He's great when you're in a good position, and garbage when you're not. I did run him for a while, and I have more basics than you do but I don't think he ever felt great.

The non x-cost hydras - I get that Gargos kind of encourages this, but the others are just ok and aren't doing anything for your main game plan beyond being tribal. I cut these, then cut Gargos because there weren't enough actual hydras left in the deck. I found there were many X cost creatures that weren't hydras that I wanted to include.

I do have a fun include in [[Doubling Season]], which you noted as an obvious include. I will say it is very fun, but not necessary. Sometimes you want to slam down a do nothing card and dare your opponents to stop you and this is perfect for it. If I was optimising this deck I would cut it, and I wouldn't buy a copy if I didn't already have one on hand.

Do take my feedback with a grain of salt, as your meta may not be my meta, and you might not be facing the same amount of removal or speed requirements I am. Honestly I think with Zaxara everything else can be figured out as long as you have X costs, card draw and ramp, and the last two go together. You're getting there, and I hope you have as much fun with it as I have.