I played both in my childhood, and I don’t think I’ve ever returned to Tooie after I beat it first time. It’s a good game, but doesn’t have that same lasting appeal as the first game.
You touched a bit on the why: the game is too ambitious. The levels are too massive and too intertwined. Smaller isolated levels fit the collectathon format better.
I remember having lots of fun with the multiplayer though, so that’s a bonus.
If you want to continue the collectathon journey you should try out Donkey Kong 64. It’s also a game that falls under its own ambitions, but in different ways. Still a good game, but nothing beats Kazooie.
Portal 64 uses a proprietary library by Nintendo (namely libultra). This library cannot be freely used, and Nintendo can sue anyone for using it without their permission. Portal is Valve’s intellectual property, so there’s a risk that Valve can be dragged in into this if Nintendo takes legal action.
Valve doesn’t want to risk dealing with Nintendo’s lawyers, so to be on the safe side they ask for Portal 64 to be taken down instead. The risks might be slim, but Valve don’t want to take any chances with Nintendo.
It was a long time ago I played it, so I’m not sure how well it has aged.
But I recommend Ty the Tasmanian Tiger. It’s a platformer where boomerangs are your weapon. Over time you unlock different kinds of boomerangs with different perks (like fire boomerangs or exploding boomerangs). I remember it being quite fun at the time.
Elden Ring really scratches that exploratory game itch for me. Every meter felt worth visiting. You never really know what awaits in the next corner. If you’re lucky it might be the entrance to a completely unique area, all with its own set of enemies and bosses. It’s a game that dares to put an optional secret area behind a secret area behind another secret area.
Even on second play through I was caught off guard by some new surprises.
Compare it to Tears of the Kingdom for example. Wherever you go, you will find the same sets of enemies and often even the same environmental assets. After you’ve visited a few of the sky archipelago islands there’s few other surprises to see there. Encountering the first Flux Construct is a fun challenge, but after the 20th one it has turned into a mindless chore.
DK64 is sometimes accused of killing the collectathon genre. While I’m not sure if the accusations are deserved, DK64 was too ambitious with the amount of collectibles and the size of levels for its own good.
It’s still a great game. I think I prefer DK64 over Tooie overall.