I have a DB9 Gen4......IMO it is TOO small, lightweight and thin for a 9MM...very snappy and not fun too shoot. And it is only 6+1 rounds. If I had been able to handle and shoot one before buying it I would not have it.
I put a crimson trace cts-1550 on my shield plus using a DPP dovetail mount and it has worked well. I also put one on my M&P40 with no problems. Only thing is if you want to co-witness you need to put a higher front sight on.
the small size of the safety on shield plus and bodyguard 2.0 make them hard to put on and off. I added some jb weld to the safety on my shield, more on top and tapered it down. Flicks off so easy now but stays harder to put on.
option 4) buy a mounting plate that installs where the rear sight is and mount the red dot to it. If you decide you like using a red dot you can THEN send it out to be cut.