It would be nice if turrets could face diagonally as well just like how railguns and now flamethrowers do.

I tried it myself and it seems like you pretty much just have to add "flags = {"placeable-player", "player-creation", "building-direction-8-way"}" to the turret attributes and it works for all the more basic turrets. The only one that didn't work was the advanced flame thrower turret, probably because it has some other needs like the pipes but that was beyond my modding knowledge.

Unfortunately, this task is beyond my capabilities.
Diagonal placement requires appropriate graphics, and I can't draw such complex images

Just adding those flags made most turrets work for me. Most turrets already have 64 directions for the smooth tracking rotations and it seems like the game handles those diagonals pretty well on it's own, you just have to tell the game it's allowed with that flag. It even handled the bounding box on it's own though to be fair some of them like the electric turret were a little jank.

The only one that didn't work was the adv. flamethrower and that can totally just be exempt. The rapid cannon turret also worked functionally but it looks like the sprite sheet for the static turret is different than the one for shooting so it's locked to 4 directions visually. That was fine for me since it's just my own world and seems to be purely visual but it can also just be exempt since that would probably require drawing more art as well for the proper mod.

I also haven't tested everything super thoroughly and don't have a ton of modding knowledge in Factorio so it could be more complicated than I'm making it. But it's worked fine for me so far