In the past all badges was considered, but ultimately rejected, seemingly because of issues like fishing RNG. Now that we have the journal, wouldn't a 100% run make perfect sense? All enemy index entries, item collection entries, card collection entries and swords?

There would still be quite a bit of RNG for sure, particularly with grinding all the ATK FX cards and getting bloxiade from camel farm, but you'd get plenty of tix having to do pit anyway, and now that hidden items are no longer hidden they're a lot easier to grind.

I can imagine this would force us to actually play a more significant portion of the game, as well as a 100% category being so common in general speedrunning I'm kinda shocked there isn't already one here. Is there any reason beyond the RNG sprinkled in?

Play a run and you’ll see why

This was in demo 1 but was removed in favor of bux%, because nobody wants to play the game for 3+ hours. Sure, other games have 100% but every other rpg game has any%. They don’t have cruel%, venom%, etc. Pit% is usually started from a new save, but in block tales it’s not. RNG isn’t the issue, it’s the fact it’s around 4 hours long at best.

The best you could get is 50% category

It's long, but that's not anything new. We already have any% fullgame runs, which are about 2 and a half hours. Most of 100% is just that and pit, with a couple extra things along the way.

It's kind of a given for every game to have an any%, or something similar to any% like 100% in the impossible quiz which is why full game runs exist. It's an rpg game, and there's not a lot of clear goals besides "beat the game asap" which is default speedrunning. 100% is different because it's an addition to speedrunning and optional.

That being said, the extra stuff adds up to at least an extra hour if I'm being generous. Pit% can be short, if you grind for lots of r-80s or bombs but in a speedrun, you won't have time to farm drops so it can take over an hour for pit alone. Bux% used to be a run so we can guage that it takes 20-30 extra minutes and all the other achievements like dx, finn, mutant, supreme mosquito, true dps, and more would prob add up to 30m to 1 hour. That's like 4+ hours for one 100% run. It isn't "a couple of extra things"

TLDR: Any% runs are required, 100% is not; nobody would run 100% optimally and in an exciting way, and it's too long. That's the concrete reason: it's boring.