faint LED glow when smart dimmer switch is off
I can't say for sure why you have that LED glow. Typically that is due to deliberate power leakage through switched hot for a switch that doesn't require neutral. However, this switch requires neutral and you have described a wiring installation where it gets neutral. So the mystery remains. But you have bigger problems.
The neutral/hot/switched hot need to be together. Not going to immediately cause a big problem in your current (pun intended) setup because LEDs don't use much power, but definitely against code and not good long-term. In addition, you very likely have a color-coding problem.
Here are the key things:

Neutral must run together with the other wires of the circuit. In some cases you don't need neutral - that's why your switch loop didn't have it. New code requires neutral at switches to take care of this problem, but old wiring is grandfathered. Grandfathered wiring takes care of the code issue until you actually need the neutral! So for this setup, you either need a new 14/3 cable to replace the 14/2 (so you can get the correct neutral) or you need a switch that doesn't require neutral.
Wherever you do get neutral, it must be white or gray (or grey). Based on your description, you very likely repurposed an unused red or black wire to be neutral. You are permitted to use a white wire in a cable as hot or switched hot under limited circumstances, but you can't use a colored wire as neutral.

Part of the problem is that historically dimmers, timers, etc. would leak current through switched hot to power themselves. That works great with incandescent lights. With LEDs, that results in either a glow or (even worse) blinking. The solutions are:

Battery - You have to change it periodically.
Neutral - You have to get a neutral in the right place (as you found out the hard way). Which is why code now mandates neutral in switch boxes.
Ground - Permitted under limited circumstances when designed properly and approved.

End result: You may need a different switch. But if the switch (this one if the problem can be fixed, or another one) needs neutral, then you will need a new cable - you can't grab a neutral from elsewhere, even though it is on the same breaker/circuit.