is it possible to make a thumbstick sort of device that you plug into an arduino that uploads a sketch to the arduino?
Not that I'm aware of but it is possible to do this via an SD card.
Here's a bootloader that allows you to upload from an SD card with only an SD module:
Here's a separate programmer device that will do the same:
The advantage over avr_boot is it frees up the 4 kB of memory used by the bootloader and it supports the ATmega2560 (which avr_boot does not).
You can follow the instructions at the link above to make your own from an Arduino or forum moderator Crossroads sells one:
http://crossroadsfencing.com/BobuinoRev17/index.html
the programmer is pretty much exactly what I was looking for. I did try googling it first... It's a bit pricey for me right now but I am glad I know that exists.
Well you can just build one yourself easily enough. The avr_boot option is the cheapest. The only thing you need is an SD module and some jumper wires. The SD modules are selling for $0.33 USD with free shipping on Aliexpress. You will need to burn the bootloader but that can be done with a spare Arduino used as an "Arduino as ISP" programmer or you could buy a 6 pin USBasp clone on eBay for $2.45 USD w/ free shipping. If you want to go the programmer route then you can use a Pro Mini clone for $1.38 USD with free shipping or a Nano for a little more.