Welcome to R2R.

My 2 cents would be to use the tank as is (even though it's not really ideal)
I'll say that from what I know, those drains are not really as well designed as they could be, precisely for the reasons already given - too small and no back up drain.
So, it seems like in order for you to get this going, you'll have to crash course everything about reefing in one go, from learning and understanding the biological system to also having to learn about plumbing your system. I think this is giving yourself a hard time for nothing and so I'd suggest taking things one step at a time. So with that in mind, either use the tank as it is, knowing that you might not want to keep it in the long run or get yourself another display tank and save this one for a little later, when you have seen more and know what the reefing "standards" are for plumbing as well as everything else. I think right now, it's not possible for you to decide what is best for you because you haven't seen enough of the hobby and I don't mean that to sound condescending, that's why I say just use it as it is to make it easier on yourself. I think if the situation was reversed and I was in your shoes, that's the kind of advice that would be most helpful to me at that point.
Actually, if you really want to get into reefing long term and you can afford it, get yourself another DT for that stand that's drilled properly. That would be specifically with an overflow built on to the back with the correct size holes for the drain and the return. I'm an advocate for as large drains as possible.
This is pretty much going to be a project for you, so you should be able and comfortable to do a little DIY yourself (sounded like you're quite able given that you said you have a CNC)
Get a list of everything you'll need to start , in terms of the DIY parts and also the other equipment that you'll need to set up a reef tank and see if that suits you, would be my suggestion. You already know you're missing lights for your tank. Lights for a SW tank are a little bit different from a FW setup. You'll probably need a skimmer to start, and also a return pump. You'll also need to dose what we call 2-part to keep your water chemistry on point. That means a dosing pump. I've basically oversimplified it, so you should research that and ask here if you need help. Research each piece of equipment so you can make the right choice, as there are different qualities and functions for each. I hope this helps you.
I'm a diver and love nature, so I would feel like something was missing at home if I didn't have any reef system set up. Welcome to the reefing hobby.
