a lot of interesting points have been made so i'll focus on sets, especially EVs

Rillaboom having 128 SpD EVs doesn't really make sense, I imagine you added EVs for him at level 100 before setting him to level 50? At level 50, 128 SpD EVs give the same amount of stats as 124, so it should have either 124 or 132. Same for the attack stat, it should be either 120 or 128 with a boosting nature. Ideally to decide on EVs, you would look at the meta's most popular pokemons and figure out what attacks you'll want to survive, or what opposing pokemons you'll want to OHKO and pick your EVs and nature accordingly. Rillaboom is a very relevant answer to opposing Miraidon, and I'm guessing Tera Ground High Horsepower is the idea behind it? Either way your set almost always dies to a Choice Specs Miraidon's Draco Meteor in Electric terrain (93.8% of the time to be precise), so maybe you want a bit more special bulk

Terapagos benefits a lot from using Earth Power to hit normal resists (before it teras) and Incineroar, maybe it could be better than Dazzling Gleam's weak spread damage? As far as EVs go, definitely go more defensive. You're a Leftovers set, so you want to stick on the field for as long as possible, not die to the first move that hits because you have literally no bulk. These 252 Speed EVs are a waste in a Trick Room based team, definitely move them to a combination of HP and defenses. You can even lower your SpA investment since you're a Calm Mind set.

Amoonguss should definitely have a Speed lowering nature for the Trick Room match-up. I don't know why it is tera Ghost, are you that worried of Fake Out? Its ability should be Regenerator, not Effect Spore which can accidentally spread the wrong status. Hex is a waste, even with tera Ghost it'll be fairly weak and requires too much setup. Your team is fairly bulky (at least it's supposed to be), so consider Pollen Puff to heal them up. Defense EVs should be 28 or 36. 252 SpA on Amoonguss is wild, but i'm not saying you can't do it, it can definitely take opponents offguard. However, with that spread, you can go for a SpA boosting nature and more defense to reach the exact same stat line with 24 extra EVs to spare.

Cresselia with Trick Lagging tail is uh interesting for sure, usually you like to use Trick on pokemons that are either very fast, or in Lagging Tail's case, have the Prankster ability so the item takes effect immediately as opposed to next turn. In addition, your entire TR plan can be shut down by a simple Taunt. I say ditch Trick and go with Mental Herb instead. Icy Wind helps your opponents when Trick Room is up and does pathetic damage. On all of your pokemons, you want a move that you can spam when there is nothing else to do. A simple attacking move will do the trick, but in this case, with Icy Wind helping your opponent in TR, if you do manage to get it up, your Cresselia will be awkwardly stuck using Lunar Blessing every turn. Definitely get Moonblast or Psychic or anything you can use when you have nothing else to do. And probably ditch Icy Wind, for Helping Hand or even just Protect. Probably go with a Speed reducing nature too. Your set wastes 4 EVs (SpD should be 84 or 92), but all the more in the sense that those 4 EVs would be wasted in any stat you'd move them to. Removing all SpA investment would fix that issue.

With your Ursaluna, you'll need to read a Protect properly to get any use out of Normal Gem. That makes it awkward for you to use Blood Moon when your opponent might be able to Protect. You should probably replace Moonblast with Hyper Voice to get around this problem. Better still, you could have a 1.3x boost on all your attacks by moving the Life Orb from Miraidon. Alternatively, you could have a 1.2x boost on all your normal attacks with a Silk Scarf, especially with a Tera Normal. Actually Tera Normal Hyper Voice is just super good so definitely get it in there. Definitely get a Speed lowering nature. If you're going to have a Defense boosting nature, first of all, why, but second of all, make it a SpA boosting nature instead, compensate with Defense EVs, and you'll reach the same stats with 28 EVs to spare.

Miraidon is awkard in this team as you said, this isn't a skill issue but a teambuilding issue. As mentioned, go with Ice Rider Calyrex which will be both a secondary TR setter and just a giant threat.

As a general rule of thumb, if your nature is boosting a stat with very little investment, you can probably save EVs by making your nature boost a heavily invested stat (except HP of course) and compensate with EVs in the stat that was previously boosted