Getting lines across when burning, and lettering looks like crap!

What kind of controller are you using? (I assume GRBL, but which version?)

If it’s 1.1f or higher, you need to set $32=1 (laser mode). If it’s earlier than that, you need to use the GRBL-M3 device in LightBurn, so it explicitly turns off the beam with every G0 move (laser mode normally manages that).

Regarding the sawtooth edges, you have likely got a slightly loose belt on the X axis.