welcome!!! first off it's awesome that you're testing the enclosure before bringing your new scale bby home; this will save you many headaches! here's some of my recommendations from what you've provided:
herpstats are fantastic products, definitely get one when you can. for one snake i'd suggest the Herpstat Intro+: all the same safety features of the more expensive models with a smaller price tag.
ditch the bulbs if you can and get a ceramic heat emitter (CHE): beeps don't like light and the CHE should be sufficient enough to bring ambient temps to target.
great idea.
just stay on top of misting and you shouldn't have much of a problem.
get two identical hides: sometimes beeps will favor one over the other for one reason or another, so to cut down on the potential of them sacrificing their need to thermoregulate over their security, you get two identical hides to avoid any stress for them.
YES get an IR temp gun; they're an amazing tool.
depending on the snake and/or where you get it from and its age, it may not thrive right away in a big enclosure. sometimes babies need THEIR needs met before your own. if your beep is not eating/losing weight, you might need to switch to a 6qt tub so it gets established. i'm not saying it's going to happen, but it is something you need to consider as it CAN happen.
you're ALMOST there!