So I purchased the m&p shield without the thumb safety. I like it but the trigger and sights have to be upgraded. The trigger is very stiff and the white dots are not well, not my taste.
Black sharpie on the back?
Sounds like I'm kidding, but in general the SHAPE of the rear sight is fine, right? The rear dots are just not fun? Simple fix.
Having a dot on the front tends to be good, for me at least. Given that it is a Shield, and you probably aren't going to be using it for 40-yard headshots, I would just suggest color in the rear dots, polish the trigger group, and just dry-fire it a lot to smooth it out.
If over time that isn't sufficient, you can always spend money on it.