As many others have already said, I would personally would not ride an e-scooter over 20mph exactly for the safety risks mentioned.
Here's a good video that explains the pros and cons of using an ebike vs e-scooter for commuting.
https://www.youtube.com/watch?v=CQWQqqG3xVg
I purchased the same Voilamart 1500W kit a year ago and I just found the wiring diagram online.
I don't think you want to split the wire from the throttle to display. Voilamart does make another kit that supports a larger display but not for the 1500W kit.
Even if you get a third party...
@Shhoom
I was recently working a similar project myself. I had an extra ebike battery lying around (52V 50Ah) and I wanted to be able to charge another battery.
In this case, since your ebike battery voltage is much higher than your SLA battery, you will need a Step down buck converter to...