Ok, so going around the youtube, I found nothing that supported the acrobatic jump-around-killing-people-with-throwing-knives idea I had in my head. So, yeah, I guess the mobility might have to go bye bye.
That leaves us at an impasse: while I saw a lot of fast knife throwing, mixed with close combat (movie stuff), nobody was exactly darting around avoiding all his opponents while making them become pincushions.
So what if we focused more on the hybrid-ity of knife throwing, something like when you hit a creature you can move 10 ft, but towards them so instead of a short range ranged fighter you basically get a long range melee fighter. If you hit the same creature you moved towards with a melee attack on the same turn, your next thrown attack against a different creature can trigger movement, so you need at least two attacks on the same enemy to move twice. Still makes you more mobile, but at least melee enemies have a chance at closing with you, and your speed wouldn't be altered in a way that you'd rather just dash (unless you're a 20 lvl fighter that action surges, but still, lvl 20 is lvl 20)
As for the reaction attack, how would that work? I would need my players to "prepare their reaction" for a trigger, otherwise they basically just get an extra attack per turn.
Also, maybe changing the ranged attack with a thrown weapon to also include melee attacks with light weapons to be more hybrid-y.
Don't know if I make enough sense, I can edit and clarify if anyone doesn't get it.