Alright, so after doing some testing, I've found a combination of items that work, in case anyone needs this in the future.

For the mixer, this 4-channel powered mixer performs exactly as advertised. I was able to play music from 2 different computers while also getting audio from my PS5 via my monitor's 3.5mm jack. Able to freely adjust inputs with no issue. I've also used the 2 channel powered mixer in the past,

That being said, there IS a substantial amount of static on the lines, considering how many different devices are involved (as a refresher, port 1 is PC audio, port 2 is 3.5mm monitor audio for use when my HDMI matrix is set to output to it, which itself has 6 console inputs and 2 different outputs, and port 3 is my work PC) there was bound to be some kind of weird feedback loop static nonsense.

To FIX that, I took a chance on this noise isolator. Used it for ports 2 and 3, only distortion I get now is when my phone is near everything and get some kind of signal. Part of me wonders if I could resolve that by just adding more isolators but it's not really a big deal to just not have my phone on the desk if it starts getting cranky.

I've mainly been working with ports 1 and 2, trying to avoid having to use the new PC for port 3 because I honestly just don't feel like it. Been getting bugged more to make sure everything works, and for my money this combination of mixer and isolator do exactly what I want them to do. Unlike with my previous 2 channel setup without isolators, I can max out every dial and volume knob involved and it be loud enough I actually want to turn things back down to preserve what hearing I still have.

So if you're looking to combine audio sources for use with 1 set of headphones, As long as you have 4 or less inputs to work with then for my money this combo should do what you need. Hopefully this helps someone in the future, since no one else was familiar with what I was trying when I was looking.

I'm considering picking up a DAC to run balanced cables in my setup. The main issue I am facing is static/hissing which I believe is coming from noise in my setup. I have to cross over a couple power cables, monitor HDMI/DP, and others with the audio cables since I have a lot of things hooked up to my PC. I believe the issue will be solved with balanced audio because the hissing noises are completely inconsistent with some days being fine and other days being straight up unlistenable.

I do use two devices (desktop & work laptop) with my speakers at different times so having multiple inputs is almost a requirement. If not, perhaps there is a USB switcher like KVM that I can use?

Below is my current setup, I would appreciate any suggestions on a DAC so that I can look for it on black friday!

JBL 305p mkii, connected to 16" TS -> 3.5mm, connected to small mixer, connected to PC & Macbook with 3.5mm -> usb-c dongle