Reddit reviews Sabrent USB External Stereo Sound Adapter for Windows and Mac. Plug and Play No Drivers Needed. [White] (AU-MMCW)

I have a feeling No. 2 might work, but why don't you try what the Ubisoft Dev said on the post?

https://www.amazon.com/dp/B00YFD2S26/ref=as_li_ss_tl?ie=UTF8&linkCode=sl1&tag=theri07-20&linkId=2209744b1c6b38f6f4f7e5a5b1716c14

It basically makes your USB port into an Audio Output. I don't know how well this works, but if the Dev says it works it probably will. I don't know how many USB ports the Slim has, but if it only has 2 then you won't be able to use guitars at the same time (though a USB hub MIGHT work, provided it doesn't suck)

About No. 1: Okay, so this is a bit tricky. The game has always allowed it to work, but since the Dualshock 4 ALWAYS runs on bluetooth even if it was plugged in via USB, there would be lag. Quoting the FAQ,

The PS4 controller is wireless (Bluetooth), and only uses a USB micro cable for power, not data. Bluetooth is a little slower at delivering audio, and we saw and felt that in our tests. We could play it, but we wouldn't recommend it unless you have no other option, because the delay over Bluetooth is noticeable.

HOWEVER, the FAQ says that since the Xbox One controller switches to the USB when connected, hence not using Bluetooth, the lag decreases and can be used somewhat. Now, the latest Dualshock 4 (the one where the lightbar is on the touchpad as well) DOES have the ability to send Data over USB (you have to turn it on).

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BASICALLY, what I'm trying to say is, in case you have the newer Dualshock 4, you can try plugging in the DS4 via USB (YOU HAVE TO TURN THIS OPTION ON, BTW) and then use the headphone jack to connect it to a headphone or a speaker. If that doesn't work, you can try buying the thing I listed above, but honestly it might give the same result since it's still using the USB port.

Also, No. 2 (if I'm not wrong) is the optimal way to go, and always has been, but the reason I wrote this wall of text is that (a) You said "buying" and I don't want you to buy 100$ worth of equipment :P and (b) in case you are beginner, you can actually live with a light amount of lag. It won't be optimal, but it won't be that big of an issue as compared to, like, your fingers hurting like hell after practicing for a few hours :P

All the best! :D

Oddly enough, I dragged this out of a drawer.. and the audio quality is maybe 10% better. Placebo effect? Idk
https://www.amazon.com/dp/B00YFD2S26/ref=twister_B01KX1R13Y?_encoding=UTF8&psc=1

I used this - https://www.amazon.co.uk/Sabrent-External-Adapter-Windows-AU-MMCW-White/dp/B00YFD2S26/ref=cm_cr_arp_d_product_top?ie=UTF8

However I'm in the uk so if you are based in the us then this is the same product on amazon.com - https://www.amazon.com/Sabrent-External-Adapter-Windows-AU-MMSA/dp/B00IRVQ0F8/ref=sr_1_3?ie=UTF8&qid=1503575932&sr=8-3&keywords=usb+stereo+adapter

sorry for the late response but thanks for the suggestion. you think something like this

https://www.amazon.com/Sabrent-External-Adapter-Windows-AU-MMCW/dp/B00YFD2S26/ref=sr_1_15?ie=UTF8&qid=1469831790&sr=8-15&keywords=external+usb+soundcard

would work? I'm skeptical because it says no drivers needed...

Except that this'll only output power, not sound. There's no DAC to be found.

Your easiest fix will be a small USB sound card like this.