There's no doubt the RCT is a potent endgame weapon. But the issue becomes more apparent when you consider the typical scenarios of receiving it.
Scenario 1: You aren't building for two-handed weapons (thus having no weapon marks in the skill) and receive the RCT as usual at level 36 while being somewhere between the Mana temple and D:50—because that's how long it takes to level to 36 unless you clean up every late-game location and/or grind and/or hit a threat room full of enemies that you need a weapon at least as powerful as the trident to kill for XP. Equipping the RCT in place of your (most certainly pretty strong) one-hander causes you to lose a chunk of DV and PV while you still cannot reasonably guarantee a kill in 1–2 hits against a balore or a similarly dangerous monster because of poor accuracy and high damage variance of the trident. A hard sell. Might work out if you're sufficiently tanky but is a liability otherwise.
Scenario 2: You're building for two-handers and receive the RCT as usual at level 36, once again in deep endgame. For some reason you haven't retrieved a much superior and guaranteed axe of the minotaur emperor (such as due to poor planning/RNG or whatever ingame time constraints), didn't receive a good endgame two-hander via crowning/precrowning, also failed to find a murderous/wicked/penetration/devastation two-hander made out of a higher metal by the time you hit 36, but have managed to locate a source of water breathing and/or means of controllable teleportation in time before the deep CoC dive. How often does this happen?
Scenario 3: You're building for the RCT. You would obviously choose to receive it at level 16 by picking Raven and just coast through the rest of the game with it. This also works for speedruns.
Scenario 4: You aren't melee, aren't Raven, and thus the trident is at best a stat stick to you. Would it outperform a weapon + shield or a decent staff this way? Highly unlikely, considering cold immunity and water breathing are mainly useful before the Water temple and you can get the immunity from the guaranteed ancient mummy wrapping already. Besides, a ranged weapon user who could use the Dx but has already lived to 36 doesn't need the trident, unless...
...Scenario 5: You happen to be a non-Raven ranged weapon user specializing in thrown spears. So why, again, have you not picked Raven?
Scenario 6: You aren't melee but are Raven, and can use the trident as a stat stick or a thrown weapon. Good for you, I suppose.
Of these base scenarios, which I'm reasonably sure encompass the vast majority of possible cases, the only ones where receiving the RCT significantly earlier (at 26–30) would make a noticeable difference are 2, 5, and maybe 1. Even then, consciously ending up in 5 suggests picking "fate decide" at character creation and ending up with a character who can still use the trident effectively even if receiving it later, or going for a flavor-based role-play build, which only happens with a relatively minor part of the populace who look for extra challenge and aren't particularly disturbed by balance issues to begin with. Somewhat similarly with 2: playing a character specializing in two-handers (thus a tanky melee type) who has the time to level up to 36 before D:50 but not retrieve the axe is just purposefully making things harder for yourself. Yes, the Maze is a chore and, at the least, requires means of remote mapping to pass through. But when the reward is the second most powerful artifact weapon in the game and a ton of other goodies useful for almost any character that makes the rest of the game a breeze, it's a bargain.
Thus it doesn't feel to me that receiving the RCT earlier than level 36 as a non-Raven would actually disrupt balance in any meaningful way. If it did, more characters would play non-caster Ravens to get the trident early. This doesn't happen. Lowering the threshold to level 28–30 appears totally reasonable; possessing the Water orb is a sensible, but not necessary requirement because it's already the easiest one to get (when was the last time you didn't have it by level 30? I usually get it at around 16–24 depending on the order of doing dungeons). In fact, I find the existence of the minotaur axe in its current state is a much more serious offender to balance, considering how much easier it is to get it compared to any other weapon of at least comparable level of power (TotRR, Needle + Sting, Foeslammer, Grod, Silence of the Dead, double-damage-affix higher-metal weapons). Slightly nerfing it to, say, (+10, 4d18+12) down from (+12, 4d20+16) wouldn't hurt its status as the most powerful weapon one could get by level ~30 at all, but at least it would make rare and extreme endgame artifacts and ultra-rare ego weapons look less bad in comparison. Right now if you get the axe, there's virtually no chance you'd get anything better until you've ascended.
There's also another option to consider: making crits themselves less powerful: 1.5x damage by default (vs. the current 2x), moving further increases to crit damage multiplier to a talent branch. This would make balancing slaying weapons much easier in general, especially with regards to ammunition.