This question is a tough one to answer and is asked rather regularly... I'd like to contribute my opinion of the matter before you search back a couple of pages for the last time it was asked or further back for even more times it's been asked. (EDIT: Please don't read this as chastising or anything. Just wanted it to be clear that this thread is great resource for discourse regarding this exact question to help you find the ultimate answer you seek)
I personally think that an extra cooling unit is a "must-have" simply for the ease of being able to keep using your Tinymight for the approximately 20 minutes it requires to disassemble, soak in a cleaning solution of your choice, wash, dry, and reassemble a cooling unit for cleaning. I only would suggest it's a "must-have" because I don't rush when cleaning one of my cooling units. I'll disassemble and allow the metal parts to soak all day if I'm feeling so inclined. I also can allow them to air dry for equally as long if desired; this is because I have an additional cooling unit and stem that work perfectly fine while the others are getting cleaned. So I find myself using a stem and cooling unit for about a week at a time before cleaning, and just rotate them as such. I could certainly get by with only a single cooling unit but the additional one makes it much more convenient to clean at my leisure.
Regarding other accessories, it genuinely boils down to personal tastes. I feel like an individual could have just a Tinymight, 1 cooling unit, 1 stem, 1 battery, and the charging cable the device comes with and be very satisfied. This is only based upon my experiences with my device and I'm (very) certain some may not have had as positive experiences with their devices (and some are VERY vocal about this, so you'll certainly hear the tales erelong). If you're a fan of vaping through a water piece than either of the WPA become an absolute necessity, assuming you're not already in possession of an RBT stem that would adequately fulfill that need. A battery charger and additional 18650 batteries are also a strong candidates for "must-have", but again if you're pretty deep into vapes then you're most likely already equipped with such things. I think a beeswax isn't a bad choice but you can just let the oils on your hands coat and age your Tinymight from simple use over time.
I hope this overly verbose wall of text at least imparts upon you that the Tinymight you receive is a completely sufficient device with what is included. All additional accessories are just that, additional accessories. They can augment your use and experience more towards what you the user are looking for, but are absolutely not essential to enjoying what the Tinymight has to offer in consistent, on-demand vapor production (again only speaking to my personal experience here, I'd hate for this to come across as ignoring the issues some users have reported) that you can literally take anywhere.