We have more than 4 friends, we have lots of controllers, now we want more games to play in offline multiplayer!

Nubs! Arena
https://store.steampowered.com/app/2337860/Nubs_Arena/

This game might only support 20 players in online mode, while local mode supports only up to 4 players. In local mode, no matter whether I enable Steam Input or not, and even when using both Xbox and PS controllers together, it only allows up to 4 players at most. In particular, the game has poor support for PS controllers—if Steam Input is disabled and more than one PS controller is connected, none of the PS controllers work at all.

I use the built-in Bluetooth on my laptop, it's an Acer Predator Helios 300.

Damn you must have a heavy duty USB Bluetooth dongle, that cant work well with that many controllers.

Hi, I'm here to share my setup, as I've seen many recommendations on this. In my case, I use eight DS3 controllers and convert half to Xinput and the other half to DS4 controllers. This way, I can play wirelessly via Bluetooth.

The programs I use are "DSHMC" and "DS4Windows."

You are correct. Without reWASD i think 4 XBox + 4 PS4 is a good mix. With SteamInput enabled you then have 8 virtual XBox controllers and without SteamInput you have a mix of 4 XBox and 4 PS4 which most of the games support.

An unexplored hack:
There is a hack to make XBox controllers to be directInput controllers but I haven't tested how well it works. It could be that it still gets filtered out from games if they still see that it's an XBox controller. The hack is to go to device manager and change the driver for "XINPUT compatible HID device" to "HID-compliant game controller".
I wonder how well it would work to have XBox controllers like this and if then works well to use use SteamInput to make them into virtual XInput devices again.

reWASD licenses:
You mention already having 2 reWASD licenses. They can be moved from PC to PC quite easily. Once you move, there is a cool-down period before you can move it again.

To support more than four Xbox controllers, in addition to installing the "Xbox Accessories Extension Driver" in the Steam client, it seems that the game itself also needs to support it. Most games still only recognize up to four Xbox controllers, and any additional controllers must use DirectInput to be recognized, such as PS4 controllers.