I do offer borderless now, but also the 3mm margin. 90% of people choose the margin. I agree it's a pain & so looking at options where the trimming doesn't need to be so precise.I just see a 3mm border as being trouble from the start no matter how good your equipment is.
I'd also see shipping frames as a nightmare.
If it was me I'd just work on adjusting the artworks so you can print borderless and reap the long term benefits.
My wife has a saying about me & printing ..." you need to get out of your own way"