You could save a few dollars by going with a GT2B Baby Buffalo, Mad Munkey or Mini thumb remote. You can 3d print a case to shrink it down.
A loop key is cheaper and more reliable than an anti-spark switch
dickyho’s shop on eBay has some good prices on pulleys and mounts
You may want to shift some of the money to the battery. At 280wh you’d be lucky to get 10 miles out of it…