Case lighting for a lighting newb

Your motherboard has internal USB headers, can you find a light strip that runs off those-- or as you said, off a SATA connector light strip or whatnot.

I think there's some adapters too but haven't tried any of them recently.

Download the manual for the B550-M HDV from the ASRock website. (it's at https://www.asrock.com/mb/AMD/B550M-HDV/index.asp#Manual). On Page 6, you'll see a board diagram. What you're looking for are connectors 13 and 14, USB_5_6 and USB_3_4. You'll probably only need one of those to drive the front panel USB ports, so you can most likely use the other one to connect a USB light strip.

I've never tried to buy any, because I think LEDs for a computer are pointless, but I would imagine that there have to be strips that will plug into those ports. Almost all motherboards have them, so they're an obvious spot to plug into for both power and control.

Again: no actual experience with any of this, I've never bothered with blinkenlights.

Another option if you need addressable RGB is to get something like an Asus Aura Terminal, a Corsair iCue commander, or one of the various DIY methods to add RGB control (with open source programs like openRGB)....

For the inside case lighting without USB, you can try molex to USB adapters. They're not too pricey, and you might find them at your local computer store. Just connect it to the molex from your PSU, and you're golden.

I know you said cheap but what's your actual budget? A quick look at SignalRGB's supported lightning controllers identifies the following:

Aqua Computer Farbwerk 360
Aqua Computer Farbwerk Nano

Asus Aura Terminal

Coolermaster ARGB Controller (Gen 1)
Coolermaster ARGB Controller (Gen 2)
Coolermaster ARGB Small LED Controller

Corsair Commander (Pro, Core, Core XT)
Corsair Node (Core, Pro)

NZXT Hue 2 (Standard, Ambient)
NZXT RGB and Fan Controller

Razer Addressable RGB Controller
Razer Chroma HDK
Razer Chroma Light Strip Set
Razer Chroma Lighting Kit

Thermaltake LEDBox

Zalman ZSync

Pretty much all of those will support light strips. Corsair and probably Razer tend to push toward custom connectors and ecosystem lock-in which you want to avoid if you're trying to save money. A lot of stuff supports Aura so that might be a good option, then they can add other stuff down the road. A quick look at the Thermaltake device on Amazon looks like it has pretty broad support too. That's only $30.

If you want glowy but cheap, some dumb rgb fans might be the way to go. You can't control the colors, but would still get the rainbow lights.

You could also get a cheap wraith prism cooler from ebay which should have a cable to connect to a USB header to control the colors and effects. A quick search looks like they're around $25.