Well at least bridge looks nicely centred using the middle mounting screw.

Are you using the bridge as a template when you start your holes?

Also, if you think you may do another build, another small but worthwhile investment maybe a portable drill guide (if a drill press is not a practical option). And I'm sure you'd find other uses for it too.

There are heaps of different ones available but I just grabbed the first one I found in a google search. There are also DIY options as well. Check out U-toob...

Here's an Amazon AU one I found: Portable Drill Guide