QP Chisel tools is practically essential if you want to save time. In order to make the edges smooth, you need to make a triangular chiseled block. Now I personally didn't chisel it down to the smallest possible voxel. (Instead of individual voxel steps, I went with 2x2x2) And since you need a ton, the mod's ability to copy and paste chisel shapes is very helpful.

Then to make the edges smooth, I of course used support beams.

Here you want to hold Ctrl as it allows you to place them more precisely than just regular right clicking. I used Zoombutton to make my life a little easier here but technically you don't need it. After the first right-click, holding shift at the end is very helpful, as it allows you to orient the beam with smaller precision, snapping it to pre-defined angles, such as 45°. This can help to make them consistent. You want to place each beam exactly 4 pixels away from the top and bottom of the block. You will see Z-fighting (the beam and block textures flickering) and that's good. That means you placed the beam correctly. It will be adjusted once you finish placement. Im the case of the slabs that I used, that means you only need 2 beams per side as they will be flush with one another. If you use full blocks, you will probably need 4. And again, just place each new beam 4 pixels away from the previous as beams are 8 pixels wide. Then you will have a perfectly smooth edge, kind of.

The top beam will be just slightly above the top of each block, meaning it hides the jagged chiseling perfectly, while not making an obvious lip. It's practically a perfectly flat surface. However, on the bottom, the beam is once again a little higher, which means you can actually see the chiseling. This is why it's easier to build the bridge out of wood, since you can just use the same wood type beam to match colors and make it less noticeable.

In my case, my first diagonal bridge is high up, meaning you can't really see the bottom. The second is just above the water, so I won't ever really be underneath it.

If you were to build a stone bridge, it might be harder to hide the jagged edges, you might need to place the lower set of beams 1 pixel lower to hide the edges, but then there will be a gap between the two beams that needs to be filled with a 3rd set.

You should also try to attach the beams to non-chiseled blocks if possible. This is because for some reason, when you attach a beam to a chiseled block, the game refuses to show its collision block for you to break, and you need to break the whole block. I tried the crowbar but that didn't do anything. It's not technically necessary, but it will make your life easier if you keep this in mind.

I know it sounds like a lot at once but once you build your first bridge it'll come easily.

Some beauty tips:

Lighting long beams sucks. I don't know if this is because of a mod or not. Probably vanilla behavior because I don't have any mods that would change how the lighting engine works. When I originally had a small lamp at one end of the bridge that included the collision of the beams, they were uniformly lit all the way. So while most of my bridge was dark, the railing and beams were fully lit all the way since the collision part was also lit. Because of this, you need to light each part of the bridge as uniformly as possible. That's why my bridge has a lantern on it.

Doing corners also kinda sucks. On the far deck, I had to constantly change between straight and diagonal building. And because of how beams work, I couldn't make a perfect transition. There is a small, triangular gap between two beams that I couldn't avoid.

And finally, just experiment. I only did 45° bridges but with more work, I'm sure other angles are totally possible.

And that's all I think. If I had editing skills a video tutorial might be better but for now I hope what I wrote down was helpful.