I would be perfectly all right if BG3 ended up closer to the 4e rules for disengagement and flanking. Would disengagement costing movement but forcing a character to take a movement penalty be a good compromise? Larian seems to feel that disengagement costing a full action is too harsh for a video game, but *some* cost is obviously correct.
Hmmm your disengage suggestion would be very similar to 3.5e/Pathfinder's 5-foot step and is certainly more realistic/immersive than Jumping. You could choose between: a.) moving away at full speed and provoke b.) backing away slowly and not get AoO'd but then you end up relatively close to the enemy. Mechanically, I suppose this version of disengage would halve or quarter your speed (min 5 feet).
However, it's not actually much less powerful than the current jump+disengage...it still gives party members the ability to ~freely (at cost of bonus action) move away from melee enemies. If you're trying to say that this slow disengagement doesn't cost a bonus action, then it's even more broken than what we currently have with Jump+Disengage.