Before starting this project, I searched through Tindie for projects similar to my idea.
I did find a few options which could technically work, but they all seem to be missing one or two key features.
WS2812 LED Strip Driver PCB
This project was interesting because it's designed specifically to drive some LED strips without much fuss. Unfortunately the boards are impossible to mount due to lack of mounting holes, the "kit" doesn't include any components and there's no schematics or other useful information.
LED Data Extender/RS485 and RJ45
This seemed to be exactly what I wanted, except it doesn't have a voltage regulator so it can only be used to transfer a low current data signal. It's essentially identical to that el cheapo board with an RJ45 for data. I like that it uses a simple dip switch to choose between RX/TX (I might borrow that idea), but the lack of ability to transfer higher voltages (ex: 12V, 24V, ...) was a deal breaker. I think the module is not too pricey, even fully assembled I do think it should probably be priced at $10 or $15 for a pair instead of $15 for one. RS485 Stick I'm still a bit confused about what this is. RJ45 to USB? Weird but cool I guess.. The description says it can be used to transfer power and data but it doesn't even have any fuses or voltage regulators. I do like the breakout pins but this is also not designed to be mounted anywhere and reminds me too much of el cheapo at a much higher price point. This module is interesting (because it's weird) but it's sadly not professional enough and lacks important usability/safety features.
RS485 HAT (daisy-chain, bus powered) Raspberry Pi I saved the best for last. I like this because it literally does everything I want, and it even supports full-duplex communication thanks to the larger transceiver. Unfortunately it has two flaws which I couldn't overlook: 1) it's designed for a RaspberryPi, instead of being a standalone module. 2) it's really expensive. It uses the de-facto standard 40-pin RPi header, but the RPI pins are then completely blocked off, so you can't even stack another shield on top of it! Overall I think it's well designed but that IC and voltage regulator are pricey, and that 40-pin header leaves you with a lot of hand-soldering (2x more solder points that my board). Conclusion
As you can see, there are definitely many solutions to this problem, including some I didn't mention. But for my needs they were all lacking something important. Hopefully I created something that will be useful for others as well.