steeldragons
Steeliest of the dragons
However, if you have a stronghold of spellcasting cultists (er, pretty much my current scenario), then you have a compounded problem of recurring enemies with spells. Actually, innate spellcasting & spellcasting occur frequently enough in 5e for this to be an issue. And your page count increases significantly if you include multiple instances of monsters like...
Well, yes it would/does...and yes, 5e spellcasting.magical ability is ubiquitous...I guess, without giving it too much examination or thought, my initial response is "them's the breaks." That's what this edition of D&D does/uses. Ergo, there're going to be many more lines of spell/ability text. The "abridged" stat blocks are only going to be able to be so abridged. The rest is editing and layout.
I see you already took the Oni size suggestion.

Then, I'd say, layout what you can, without making things too crowded, to save lines where you can. Just off the top of my head (granted, Enworld viewing is not the same as in print spacing) go use the next line until you've used at least two columns.
Oni: Large giant LE; AC 16; HP 110; Saves Str +4, Dex +3, Con +6, Int +2, Wis +4, Cha +5; #Att 2 glaive, Large +7 (10 ft), 2d10+4 slashing / Medium +7 (5 ft) 1d10+4 slashing; Spd 30, fly 30; Senses 14 (darkvision 60 ft); Lang common, giant; Special change shape, magic weapons, regeneration (10), Innate Spells (save DC 13)At will: darkness, invisibility[ . . . . . ]1/day: charm person, sleep, gaseous form, cone of coldCR 7 (2,900 XP); Monster Manual 239.
Flame Mage: Medium humanoid any evil; AC 14 (with mage armor); HP 38; Spd 30; Senses 15; Lang common, ignan, jhatab’enar; Resist fire; Saves Dex +1, Con +1, Int +6, Wis +5; #Att 1 fire bolt +6 (120 ft), 2d10 fire; dagger +4 (5 ft), 3 (1d4+1) piercing; Special geased; Spells (save DC 14, attack +6)cantrips control flames (EE), fire bolt, light, mage hand[. . . . ] 1st (4) burning hands, detect magic, mage armorCR 6 (2,300 XP); p. #.
2nd (3) flaming sphere, scorching ray, suggestion [. . . . . . . .]3rd (3) dispel magic, fireball, haste
4th (1) wall of fire
There. Just saved you 3 lines in those two descriptions alone. Oni x4, there's 4 lines saved, there, alone. From the number of cultists and special whatever-they-are-casters, probably could save several ten's of lines.

(Ignore the dots in brackets, I wasn't sure how to supply the spacing [somewhat] evenly on Enworld without going into html table rows and all that mess. But you get the gist.)