So much information has been shared and i agree with the others. Adding in on your other comments:
Digital Aquatics went out of business and their products are no longer supported. That is why i sold mine and only have the heart left for temp and heater control. I am currently building a reefpi for the same reason and then will sell the last of it off. I am also adding top off, which is something i think is critical and love, but upgraded to an unreliable unit that i then sold back off, hence reefpi exploration.
Using the Apex outside of that is personal preference. I personally will most likely never own one. I will also never talk someone out of it. To me its like arguing about the skimmate shut off switch. Sure you dont need it, but i like it shutting off without spilling back in my tank when i get a surge for some reason, or am just to lazy to check it in time.
I personally want an Apex or GHL for automatic water changes, but for the cost, its not worth it to me. I spend a lot more money in corals and building my new business. If i had the justification to spend the money, i would have already because they add so much opportunity for fun or control.
You already spent the coin, so its a sunk cost. Dont spend more to replace and do what it can do (DA RKL) because if you ever want to expand you will just spend more money. I would personally keep it, use it as desired and maybe learn a lesson about jumping into tech to early in your adventure. If you sell it and buy something else, you will most likely regret it as the new option will be far more limited and you will come far from breaking even on the money. Spend 800, sell for 600, spend 100 net gain $500 (maybe). Regret that you now have to buy another brand for ATO ($130) and Heater safety ($100) and you are down to netting $270 and you have multiple apps or buttons. Plus a good power switch... see where i am going?
Good luck bud!