Ah, well I wasn't aware of that; that does seem a bit scattered. At least this was refined in later re-releases (as far as I know, or at least to the point where I didn't notice any major issues).
But the core philosophy and gameplay loop of 1-3 is very appealing to me. I like the rhythm of going from the town to the maze and back; deciding when to return with treasure, and when to push my luck for a higher potential reward.

Hm! As far as level 5-8 are concerned, well, I simply don't agree with your assessment. Before I had actually played the game I had always heard from people that 5-8 feel like filler levels, and so I was apprehensive to explore them myself. I was pleasantly surprised then, when I found out that they aren't filler at all- nearly the opposite.

See, floors 1-4 are the tutorial. 1-4 really is just the "Proving Ground" to reach the real dungeon of the game.
Once you get past that, the dungeon gets a lot more tricky, and you have to put all the mapping tricks, combat strategies, and general survivability skills to use.

A common complaint I hear is that the game is just mindlessly grinding on Murphy's Ghosts to get to a high enough level to plop down and kill WERDNA, but I think that is missing the entire point of the game.
There is absolutely no needless grinding required if you fully explore each floor. By the time I had gone through all of the floors, all of my characters were level 10, and I managed to kill Werdna after 2 tries. I didn't even have the highest level spells at that point.

And that isn't mentioning the fact that loot is gained by floor level; the deeper you go, the better the treasure gets.
I've come to view 5-8 as the real meat of the game.

This actually reminds me of the Bard's Tale series.

In Bard's Tale 1 and 2, the gameplay loop you mention clearly applies; you go to the dungeon, decide when to return with treasure and XP (or just because you're running low on spell points), sell extra loot, and repeat. However, what's interesting is how Bard's Tale 3 breaks this:
* There's no shop, so no place to sell extra loot.
* There's no Roscoe's to easily restore SP; however, there are Harmonic Gems, which are a common item (except in the broken 16-bit versions) that restores the user's SP fully.
* Also, save anywhere means you don't need to return to town when the playing session is done.
So, in BT3 you only need to return to town when you have the XP needed to level up and actually wish to do so.

By the way, what do you think of Wizardry 4 and 5? 5's gameplay loop feels just like 1-3, while 4 is its own thing and one of the most unique RPGs I've ever played.

The point isn't so much that the levels would be filler, but rather that they're completely optional. If you win the battle on level 4, you get access to an elevator that takes you straight to level 9, allowing you to skip all those floors. If you choose to take the stairs instead, I believe there's no path that would take you to level 9; the only way to go to level 9 if you take the stairs is to teleport, and if you can do that, you're high enough in level to beat the game.

Incidentally, the NES version completely changed levels 5-8.

(By the way, there is one time, playing with the Identify glitch in the Apple 2 version, that I teleported, in battle, from level 1 to right before Werdna. A lucky MAHAMAN, and Werdna was dead.)

[Wizardry Gaiden 4 SPOILER follows)
One other interesting note: Wizardry Gaiden 4 has a dungeon whose first 4 floors are almost identical to the first 4 floors of Wizardry 1, except for a couple things. You can't enter the Murphy's Ghost room (at least, not when you first get to this point, and when you finally do, it's not a Murphy's Ghost fight). The square where the stairs down from level 4 would be is instead solid rock; the square right before has what I believe is a warning, but if you go through the door, your whole party will perish due to being in rock.

Actually, unlike in the Bard's Tale series, loot is determined by the encounter formation, or at least by the lead monster in the monster formation.

Actually, in Wizardry 2 you *must* have the MALOR spell to go past the first level of the dungeon. If you try to just walk to the stairs, you'll find that there's a teleporter that will take you back to the square you came from so you can never actually reach the armor or the stairs. As a result, you have to level up a character to level 13 as a mage (or 22 as a samurai, or 25 as a bishop) in order to really play the game.

Fortunately, you don't need to rely on RNG; In Wizardry 1-3 (not 5) each spell level has one spell that you are guaranteed to learn if you don't already know a spell of the level but are high enough in level to learn one. In the cast of level 7 mage spells, that spell is MALOR (except in GBC Wizardry 1-3 where it's TILTOWAIT, which can be annoying in GBC Knight of Diamonds).

NES Wizardry 2 (3 in Japan bur it's really Knight of Diamonds) is different; you can get past the first floor just fine, but you can't walk to the square with the gauntlets (on level 5) without already having the gauntlets in your inventory. So, you still need the spell, but not until the end of the game rather than near the start. (Note that this version also has a fetch quest that isn't in the PC version.) The SFC and GBC versions are based on the NES/FC version, but the PSX/Saturn version is *not* (and I note that the default party here is only level 10, so not enough to get past level 1).

Of course, thinking of Wizardry 2 I still think of an exploit that allows you to grow your character's level exponentially with some patience.