You can reduce the DMG monster CR guidelines, for CR 1+ creatures, to a pretty simple formula.
Add up at-will damage per round, divide by 12 (ignore miss chance). Add up HP, divide by 32. Add up "one-shot" damage above at-will damage, divide by 36. Add the primary stat bonus for attacks divided by 4. Add AC divided by 4. Then subtract 4.
That gives you the CR for a creature with no status effects or special abilities.
The DMG provides rules for handling legendary resists, advantage, invisibility, magic resistances, area effects, etc.
While this system doesn't seem that similar to the DMG rules, I find it quicker and simpler to tweak monsters.
Suppose you have a creature that that has 252 HP, has 22 intelligence, makes 3 Int-based attacks for 3d12+6 psychic damage against AC, and 1/encounter can deal 2d12 (save for half) thunder damage when damaged to everyone within 200' (when "bloodied"); save DC is int-based. It has 17 AC (natural armor). It has magic resistance and 3 strong saves.
What CR do we get?
+6/4 (Int) + 17/4 (AC) + 76.5/12 (at-will damage) + 39/36 (AOE extra damage). MR is worth +2 AC (+0.5 CR), 3 good saves is worth +2 AC (+0.5 CR).
Total is 10.2, so CR 10 monster. From its CR, it has a +4 proficiency bonus. So its (Int-based) attack bonus is +10, and its save DC is 18.
(For fancy stuff from the DMG, if it boosts "defensive CR", add 1/2 that amount to total CR. If it boosts effective AC, add 1/4 that amount to total CR.)
For monsters above CR 20, you have to divide the CR by 2 and add 10.
This doesn't help with sub-1 CR monsters at all. But it does make higher level monsters really quick to measure.
For sub-1 CR monsters, I'd advise reskinning. Because they should mostly be a pool of AC, HP, a single attack with moderate damage, and maybe 1 special trick.
Taking an existing low-CR monster and reskinning, possibly tweaking abilities, is probably best for those cases. If you are doing anything seriously weird with sub-1 CR monsters you'll have to do real playtesting anyhow.