Since you have the Apex already it makes sense to buy the DOS. Unfortunately, the DOS is only a 2-pump model, and that may or may not be enough dosing heads for you depending on which method you want to employ.
I use an old style grey and blue GHL Stand Alone Doser ("Doser 1"). It's great. The old model that I have is prone to losing lines of pixels in the screens, but replacement screens are only CAD$80, so that's not a big deal. I imagine they won't have them available for much longer, so I have an extra one on hand just in case. I haven't used the new style GHL, but the Wifi capabilities may be handy for some people.
I own a maintenance company, and most people don't see the value in buying German products, so most of my clients use Kamoer or Bubble Magus dosers. So far, (touch wood!) I haven't had any issues with these dosers. One draw back I've learned though is if you leave them in the "setup" screens it will not dose, AT ALL. So you have to make sure you're back in the main screen anytime you adjust anything. As long as you know this, it's not a problem.
Years ago I had an SPS tank with BRS Drew's Dosers. I still have these, and they are still in working order. They don't have controllers built in, so they need digital timers or an aquarium controller to control them. I haven't used the BRS branded ones.