Any comments on your progress since you posted this?
I also own a Dell 15-3567 as well, I5-7200U, Touchscreen, currently Running 32gb of Crucial ram no issues but I also never had a memory issue prior to the upgrade either. The 2tb Crucial ssd was best upgrade I ever did and really breathed new life into this laptop after the original 1tb mechanical HDD failed entirely too quickly and the 2tb mechancal replacement started acting a bit wonky after only a few months installed as well (still plugging along in the optical bay caddy now).
Sure you must have seen the post on the Dell forums https://www.dell.com/community/Inspiron/Dell-Inspiron-15-3567-Backlit-keyboard-upgrade/td-p/7508135/page/2
that says the I5 motherboards are all compatable with a backlit keyboard & Can tell you Amazon has a lighted keyboard at this link
https://www.amazon.com/dp/B06XCRW2BQ?psc=1&ref=ppx_yo2ov_dt_b_product_details
that will definitely fit the laptop, but, like you I have not yet found a jack on motherboard that will take the backlight ribbon, but I gotta figure we could probably figure out a way to bring power to it one way or another with a little noodling (probably by tapping a usb rail).
Saw somewhere that someone said the Trace on the I5 motherboard where the socket for the keyboard light is supposed to be installed at is supposedly identified by a "BL" marking, but I have not pulled my motherboard out to look at it under a magnifying glass to check it to see if I could try & identify it.
Recently bought this Intel AX200 WIFI 6 bluetooth 5.0 card for use in an older HP 15 AC121DX, I3, Touch laptop that never had internal bluetooth,
https://www.amazon.com/dp/B07V86Q3WP?psc=1&ref=ppx_yo2ov_dt_b_product_details
but it wouldn't work because HP apparently used BIOS Whitelists to block unauthorized user upgrades to the laptops they own to keep them current and extend the planned obselescence cycle. Shame as it really was a decent light 15" laptop for the money in it's day.
Might try to throw the AX200 in the Dell to see what happens to it's performance and see if HP blocked the QCA9565 2x2 WiFi/bluetooth 4 card that the Dell came with just for giggles to see if I can kill two birds with one stone.