joycontrol VS Switch-Fightstick

Uh... There are DIY alternatives. Check this out.

Don't know for Arduino but I've had successes running joycontrol in the past on a laptop.¹

If I understand your requirements properly I think you would want to make some kind of relay device, the overall goal being to receive a single controllers bluetooth inputs as a host, and then to broadcast them to each BlueRetro receiver. The easiest route especially with tinkering in mind would probably be to use a Raspberry Pi Model B, making use of one of the open source projects that can emulate Switch joypads, such as: https://github.com/mart1nro/joycontrol

Last night, I was playing around with stuff like joycontrol, joytransfer and splatplost (repos here: https://github.com/Victrid/splatplost https://github.com/mart1nro/joycontrol https://github.com/lowlevel-1989/joytransfer), and I guess maybe I got frustrated with the virtual environments (as things were not working as intended) and decided to remove them and run things without it, as well as deleting a pycache in the joycontrol folder that was root owned. Bad idea!!

The library used was joycontrol which uses bluetooth to connect.

No. You can even connect and emulate controllers via bluetooth with joycontrol https://github.com/mart1nro/joycontrol. As for the switch fightstick thing, you need the hardware but it is just like using any other USB to the switch

The source code I based my implementation on is found on this GitHub repository: https://github.com/shinyquagsire23/Switch-Fightstick/ but I had to make a couple code modifications to fix some bugs that I found while using it myself.

For Splatoon 2, you can do this with an Arduino Leonardo.

Splatpost printer can easily be repurposed to enter those inputs for you if you have a microcontroller like an arduino or teensy 2.0++. A Titan two can easily do the same thing. This controller also support macros so it might also do the job as long as there's a way to make it keep repeating the macro.

It's a piece of software that turns teensy or arduino boards into peripherals to upload images into splatposts one pixel at a time. If you see artwork in the plaza that's pixel-perfect, usually it's because they're using some kind of pixel plotter, rather than doing it all painstakingly by hand.

You can also make a 320x120 image in your favorite pixel editor and use something like this thing to transfer it over. Tried it some while ago, it works pretty well, just takes some 20 minutes or so haha

I'm an absolute newb regarding Arduino (not so much for computers and programing), but there was a project that I really wanted to do: https://github.com/shinyquagsire23/Switch-Fightstick

Some people use this tool, but it has extra hardware requirements. Others just work on it pixel by pixel.