Sandra writes about Challenge Ratings in D&D:
With both a request and a bit of philosophizing.
The request is for monsters and groups of monsters by CR. This should be pretty easy. If you're using 5e, you can use 5etools and/or Kobold Fight Club to save logical groups of monsters. If you want, you can store these in a table, as with Abulafia or an Abulafia-like table automated via html.
Abulafia-style random tables in html
I'm less familiar with tools for other systems, though for popular ones like 3.x I assume they have to exist. (I do know that for 4e the Dragon subscription let you scale *any* monster to *any* CR, but I suppose that's lapsed?)
But she also muses:
Now, it’s my philosophy that 3.x, PF, and 4E got it wrong with over-reliance on the CR system. Having the DM “serve up” encounters makes the party’s deaths the DM’s fault and the party’s victories the DM’s fault too. Instead, I want a pull model: a world full of explorable danger that the party can explore as they want to: push into dangerous places in search of higher rewards. The XP curve is a brilliant invention in service of this model. So, the tier&level recos should be just a vague guide for the DM (place more dangerous stuff further away). The idea isn’t to serve one dungeon at a time to the party and say “it’s a level five dungeon and you’re level five so STFU”. Instead, just use this mad libs / paint by numbers system to make a bunch of dungeons and place them and let the players explore as they wish. Don’t tell them what level this system claims something mathematically is for (until afterwards, if they ask. “Yeah, you defeated a dungeon for level six chars, that’s great for a bunch of level threes.”).
tbh I think that this is something the OSR gets right: you don't need CR, you just need (1) diegetic telegraphing of when danger is likely to appear and some possibilities for avoiding it, (2) easy character creation. These together mean you can put whatever would logically be somewhere without thinking too hard about whether it's "too hard" or whatever.
This has some other nice effects: take the logical next step of nixing XP for monsters and you can also nix CR, which means you can nix "monster design" as distinct from let's call it "monster representation" - that is, making sure the monster stats (insofar as you even need to invoke them) faithfully represent the fictional creature that they're there to represent.
but it looks like you solved most of this already!