joycontrol VS splatplost

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

You can do this without specialised hardware - just a computer with Bluetooth and a flash drive you're willing to use as a boot drive - via splatplost (https://github.com/Victrid/splatplost).

Like u/T-RD mentioned, you can use a Teensy with a fork of Switch-Fighstick or you can run Splatpost on a computer to emulate a joystick like u/DrunkCloudHoward mentioned. They both turn an image into a Splatoon post so you can use your pre-made art or meme as a Splatoon post.

There's Splatplost, although I've never tried it myself.

The key thing is that the person who developed this created a flashable iso that handles dependency hell for you - https://github.com/Victrid/splatplost/blob/dev-v0.2/docs/image.md - this makes installation a lot easier, you just flash the provided iso to a usb drive and boot from there.

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!!

Probably just plading pixels. Something like this.

The first one is most likely done with the Splatpostprinter: https://github.com/Victrid/splatplost (tl;dr: you buy a small chip, install the program and plug it into the Switch USB and it pretends to be a controller and draws automagically)