What about the hEX S made you choose that model? Are you planning to use the SFP port? As has been stated by @MickeyT the PoE out on the hEX S passive, but I am not sure that it couldn't be used with the UniFi6 Lite AP, which requires 48V. The hEX S can be powered by 48V and I assume pass it through to the output, but the power supply it comes with is 24V. So you should read this thread
My heX S can't power up Unifi 6 Lite. I would just use a PoE injector on the between the router and the AP myself. About the only advantage of using passthough is that you can power cycle the attached AP with a command. But you shouldn't need to power cycle the AP, it you do, that's only treating the symptoms, not the problem.
The RB750Gr3 and RB760iGS are quite similar except for the SFP port.
RB750gr3 vs RB760IGS?
And they are both similar to the ER-X (all are based on the MT7621A MediaTek chip). Any of the above would "work", but none are really great if getting the last bit of performance out of your high speed internet connection is important to you. (But is it really? Or is it just for bragging rights?)
If you are familiar with Linux, the ER-X (vyatta based) provide access to many of the linux utilities, where ROS gives you a predefined set of tools. ROS has many more knobs to turn, but very few blade guards, so you will be much more likely to make mistakes with ROS.
Because you said this: "I'm willing to learn how to get what I need, but I won't learn enough to tinker with things outside of what I need if that makes sense." it does not seem that leaning networking is your goal, it seems that you want something that is more like an appliance. And for that reason, I don't think that ROS is the best choice for your requirements. Even the ER-X would be easier to configure, because you can find free documentation like
this that will guide you step by step. But finding an ER-X in stock at list price now is nearly impossible.
About the closest thing to Mike's book for MikroTik are the links in @anav's
New User Pathway To Config Success, although that isn't specifically targeted to a specific situation. But it would be a good place to look before deciding if the level of detail there is sufficient for you to follow. Even Mike Potts' pdf book assumes knowledge of networking fundamentals.
Other things to consider: pfsense on a dedicated network appliance. Even OpenWRT on another router ARM or x86. (even a Raspberry Pi with an external vlan-aware switch running OpenWRT) You can find info about these on youtube.
But if you want to learn networking, using MikroTik will force you to learn.