For me, I would focus on leg drive last. My 12YO can probably hit 40 just playing catch underhand. Maybe 43-45 by focusing more on arm speed. Her top speed via a walk through is 48 or 49. For a beginner, a strong leg drive might help with speed, but may not help with mechanics. I see lots of beginners at the JV and 6-8th grade level that leap out, but are still lobbing the ball in.Video this season absolutely confirmed they do NOT have good basic mechanics. Lol. But it sounds like you would absolutely emphasize I/R over body awareness, etc., for lack of better term? I just ask because in learning myself, I suppose I hammered home I/R pieces for a long, long time, before moving into drills, but I felt like I learned more/got more bang for the buck by learning drills to emphasize leg drive, etc., versus lock it in, etc.
To borrow your term, good basic mechanics = effortless velocity. You're clearly not ready to give up on them - so keep trying different cues with different players. I like the Boardmember video where he just holds the end of a bat, and swings it back and forth - palm up behind him (9:00), palm down in front (follow through past the belly button). That's the feeling you're looking for. If the lock-it-in drill (with a ball) isn't clicking with them, try it with a bat!