Symphony of the Night is imo just not very good to begin with. Great art, great music, but the game core itself is quite shallow. Both prequels (which are an entirely different genre so they cannot really compare) and many of the sequels are far superior.
Also, the reverse castle is the cheapest artificial lengthening device ever conceived.
Completely agree with you. The prequels peaked with the VERY underrated Bloodlines/New Generation if you ask me. Rondo of Blood/ Dracula X being a very close second. As for the sequels, i personally do not care for Circle of the Moon, which i found very bland. Harmony of Dissonance has to be my fave in the GBA trilogy. Not gonna remove merit from Aria of Sorrow ofc. DS trilogy is REALLY where the Iga-vanias shone the brightest if you ask me. With ever increasing quality even.
My opinions, if you're curious:
* Symphony of the Night: Good game, but overrated. Has a few major flaws, like random stat growth, the fact that reaching the level cap is not feasible (eventulally every enemy gives only 1 XP), and the fact that the game is too easy most of the time (there's multiple ways to break the game). When the game isn't easy, it's not very fair (no invincibility after getting hit makes spikes a serious issue, and Beam Skeletons are nasty when you first encounter them). There's also the problem of needing to press fighting-game style button combos to use magic.
* Circle of the Moon: The first Castlevania of this style I played (not counting CV2). My favorite feature is the bonus modes that unlock after beating the game: Replay the game, but your stat growth (which is no longer random, by the way) is different, and there may be another rule change depending on the mode. Downside is the reliance on random drops for equipment and spells; a typical player, unless using a certain easy glitch, is unlikely to see most of the spells except on Magician or maybe Thief mode. (Thief mode is my favorite; normally rare drops become absurdly common.) Also, some spells are a pain to use because, again, you need to use fighting game-style button combos.
* Harmony of Dissonance: Perhaps less interesting than the other games of this sort, mainly because you are stuck using a whip for the entire game. The two castles were not really different enough for the mechanic to feel interesting. This game has one of my favorite spells; just try to summon without a subweapon. One downside is that the same issue fro SotN of enemies only giving 1 XP at high levels still applies here (though at least you don't have random stat growth).
* Aria of Sorrow: The soul system is really interesting here, and provides many interesting possibilities, which can be fun to play around with. My biggest complaint is that the secret character can't play through the final area of the game.
* Dawn of Sorrow: See Aria of Sorrow (except for my comment about the final area). This game has the best extra mode (accessed after beating the game) of any of these games.
* Portrait of Ruin: Has the best music of all these games, and also has some nice worlds; plus, you can switch between two characters, one of them female. It also has my favorite difficulty setting, Hard level cap 25 (which was unfortunately dropped in the next game); that mode keeps the second half challenging, though I think the last two bosses might be too much, especially if you're playing a certain extra mode.
* Order of Ecclesia: I haven't played this one as much, but I have beaten the game on normal. Female main character, intereseting weapon options (IIRC), and the game is actually not that easy, even on Normal. I don't really like the extra mode, however, although I didn't play much of it.