Typically, the small guys who throw gas are great mechanical models. the more you learn to move like the little guys, the harder you will throw. I'll be comparing you to sonny gray's mechanics from a couple years ago, which were perfect until this season.
I'll start off at a basic level, and as your biomechanics improve I'll go deeper in with analysis.
First thing I did was take you into your leg lift.
Notice the different between you and Sonny? He is transferring his momentum a lot earlier and quicker. Once his lift leg comes up, SG (Sonny) has his hips moving to his target forward and down at the same rate. While you do have some shift in momentum, I think it can be vastly improved. The better you get at this, the easier it will be to get force going down the mound.
To improve this, I would recommend thinking about pushing your front hip to the catcher as your leg comes up. With any advice I give on coaching cues, I would advise looking around for other cues on doing this to see what works with you.
Next I took you to your load.
At first glance they appear similar, however there's a big difference; the direction of your force vector (your ankle knee aka your tibia).
Before your back leg either drives or rotates, your force vector should be pointed to your front hip. In doing this, it is pointing to your target and considered "linear". Think of a sprinter before they run. Their force vectors are pointing to where they want to go at the start. This allows them to push the force in the direction they want to go in. Same principle.
As you can see SG's is a little higher than his hip, although I think thats just my inability to draw a straight line and pausing. Bottom line is, is that your force vector is not linear, like SG's. That's a big issue. Just fixing that could get you a couple MPH. I'd say to set up buckets or something similar so that you go through your delivery and land on them with your butt. They should be set up so that once you land, your force vector is linear. If you do this a few thousands of times, it will go into your muscle memory and replace your current delivery.
Why is this important? It sets up SG to do something that you don't - to drive. Once your force vector is in a linear position, your back leg needs to really drive through, and extend all 3 joints (ankle, knee, hip). If you take anything from this post, be it this. A key thing here is that the timing needs to be right, or you're not helping yourself, and you're just pushing yourself off the rubber which is highly criticized. Look at videos of Sonny, Tim Lincecum, and older ones of Chapman. They all (as well as many others) drive the back leg through.
One immediate reg flag that should go off is that you see plenty of professional pitchers not driving. That's true. This is because driving off the rubber requires great power, that many just don't have. It also requires elite motor control which needs to be developed, which not everyone is willing to if they don't need to. An example of this is how Chapman has started driving less recently, simply because he doesn't need to anymore to get his ball speed where he wants. He has the strength to throw triple digits while mixing between driving and rotating, so he doesn't worry about driving.
Why should you drive through?
Driving the back leg to get the back hip "through", or rotated has many benefits.
First off, it gets more force going to the plate. This will mean your front foot will strike the ground at front foot strike (FFS), meaning more force will be sent up your body from the ground.
Second, you have faster hip rotation. This is key to velocity. Think of a propeller toy. You can not make it fly by spinning the actual top, but only by spinning the bottom piece. The same physics from your high school class applies with the force multiplier coming form the center. (I am not explaining to save space and because I am assuming its not necessary. However, if you want me to explain i will). The faster your hips rotate, the faster the ball comes out of your hand.
Thirdly, and most importantly, it gets your hips ahead of your shoulders. Here is SG at FFS
His hips are COMPLETELY open, and his shoulders are pointed at the target; their separated. Hip to shoulder separation is a HUGE part of velocity. When you drive your back leg through, you enable your hips to fully open while your trunk stays pointed at the target, effectively putting torques on your core.
Lets take you at FFS
You're already rotating your trunk. This is why you throw 76-78. You need to vastly improve hip-shoulder separation. I think you have some, but not enough. Driving the back leg through will get you started on hip to shoulder separation.
Luckily for you, I think there is something you can do that does not involve the lower half that will help hip-shoulder separation a little. Not as much as driving the back leg, but it is still crucial. Its your arm cocking. At FFS your arm is already cocked. It shouldn't be. Look at Sonny's, its only partially cocked. Here is a link to something I wrote about this a few months ago; https://www.quora.com/What-is-the-proper-arm-action-when-pitching/answer/Daniel-Ryan-53 Its definitely enough to get you started.
When your arm is cocked early, your body says "okay! time to go!!!" and the trunk rotates. Late cocking will help prevent this.
And the final major issue I see if your front leg. It needs to extend during MER. In my experience, many coaches will say that if you're a bigger guy you get a pass on this, but I disagree. Front leg extension is HIGHLY, HIGHLY correlated with ball speed, and you're doing yourself a great disservice by neglecting it. Another thing you may have been told is that you're pretty big and that its for small guys. Great news! Sonny weighs more than you (190) and extends it, as well as other guys over 200 -- so its definitely not impossible for you. Now that you know that you CAN do whatever i have been ranting about, lets discuss it.
Here's sonny in MER
Notice his front leg.
Now here's you
Notice the difference in front legs?
When you throw, think about driving the front leg into the ground as your throw the ball. Practice driving with the back foot so that you have a full stride, and then drive the front leg into the ground as your throw. Do this when warming up and as your progress. I naturally have a HORRID front leg which went from bending more and more as i went forward and swiveling to looking like yours in about 2 pens from practicing what I said.
These are drastic changes, I know. I hate to say it, but you're not going to ever hit 90 without them. Maybe not even 85. While small changes may seem easier and less intimidating, they yield less results. As I previously said, these are basic, and as these aspects improve we will go more and more in depth with smaller tweaks. What I just gave you is probably 2 months of mechanical work. All I can say is to grind it out and let results speak for themselves.
(going to steal a line from LankyLefty here)
as you begin to get into the world of pitching velocity, you will find a lot of contradictory information. Most of the info you will find is just plain inaccurate, and unsupported. The best way to sort through this is by looking at the countless studies done by ASMI and other institutes. I really recommend reading up on them. google scholar is great, just search what you want and studies (and only studies) related to it will come up for you to read.