For the Particle System trail, there's a "Default-Line" material you can select under the Trial module.

You can also make your own custom trail material (put in your own texture) if you Create > Rendering > Material and set the Shader to "Legacy Shaders/Particles/Alpha Blended Premultiply" (which is the same as the default material as the Line Renderer).

Recreated the fire with VFX Graph, which had a bit of a learning curve, but I managed it.

For VFX Graph, there's an "Output URP Lit Mesh" and you can use a Quad, which allows lighting to affect the particles.

Also VFX Graph's "Color over Lifetime" + an exposed "Gradient" property happens to only have a linear color space (unlike Particle System's gamma color space?), so I had to modify my original color curve from my Particle System fire to match.

A little troublesome, but overall, definitely can see how much more powerful VFX graph can be. Still prefer the Particle System for its ease of use, however.