There’s a lot to be said for using what you know and are comfortable with, when it comes to safety/monitoring systems.
Setting up the InkBird controllers isn’t very intuitive but also isn’t difficult if you commit to figuring it out. I have 7 of them in use and have had no issues with any.
For heaters in medium and large tanks, I’m a fan of having 2 temp controllers per heater (can be InkBird+onboard thermostat or InkBird+Apex) and 2 undersized heaters to give multiple layers of redundancy against both fail-on and fail-off scenarios. And then TEST failure scenarios as best you can and make sure it does what you expect. Would definitely have the heaters in the sump, but that’s aesthetic.
The issue with 2 undersized heaters is you don't have redundancy and you can't really effetely lead/lag them (that is mechanical speak to tun them on/off in succession). Mentioned lead/lag above but didn't didn't go into detail. Its like controlling an RC car with only a on/off control for turning left/right vs a variable control, or docking a boat using the on/off of the engine vs throttle.
The failure mode if you need both heaters to get you to temp, when one drops out (and it is actually cold) your heating power is now 1/2 and the water temp will fall to an unknown lower point based off the temp of your home, size tank, etc. You will need to address that quickly meaning your vacation is cut short, etc. OP has 3 heaters, so assuming they are all sized at 50%, he is good. Using equipment OP has, If you have 2 full size heaters with different set points and the primary heater fails, you can be notified if you are running say 1 or 2 degree under normal vs way under normal. That gives you plenty of time to fix a heater with no damage and you can chill in Hawaii for a few more days. PS, I Design and Build Commercial Buildings with heavy mechanical systems for data centers, life support systems, labs, large assembly areas like Entertainment Venues so If I use jargon that isn't clear, let me know. Also take my suggestions as Best Practices which does not always mean practical or in budget in a home fish tank. Everyones risk tolerance is different.
I see your often stated point that 2 smaller heaters will "save" the tank if a heater sticks on. It won't if the heater sticks on in the summer so that can be a false sense of security in a way. Having 2 systems to shut down a heater is really what you need in that instance which means you can safely use full size heaters, or three 50% heaters, etc. You have an Apex to take care of that so You are good, but proposing the alternate argument for those without a secondary back-up and just using a cheap heater + temp controller. Once you get an Apex or similar you have a lot more options such as power monitoring, etc.
Notification of a failure is actually the most critical, because if you don't know the system failed, you will just be operating on a back-up as normal until the secondary fails. That is the best reason to lead/lag your heaters if you don't have something like an APEX with power monitoring. This can tell you a heater has failed and give you the opportunity to fix it before the fish tell you it failed.
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