I think that a potential solution to the problem of not having enough racquet drop could simply be to get your racquet up behind you earlier on in your windup.
Instead of the racquet being either at your waist of below it when you toss the ball, experiment with starting into your motion with the racquet already up behind you. It may feel strange (hey, it's different) but I'd expect that you'll have an easier time getting a smooth, full release through the ball with a racquet drop behind your back shoulder. When the windup is further along before the toss goes up, it's usually easier to complete a proper swing at a serve with good, un-rushed timing.
Also, I don't think you mentioned any lingering issues after your back injury, but if your range of motion is limited or something, let us know. Everyone needs to build a service motion that's repeatable. This is a huge issue with younger players who shouldn't be taught to serve with excessive back bend that will put them at risk as they get older and less flexible.