So, first off, I feel obligated to say that as appealing as the advanced firewall route may sound, it is going to take some serious work and learning on your part, and I don't want to discourage you from trying, but I also don't want to send you down the wrong path for your needs if a much more turn-key setup will suffice.
A simpler solution I forgot to mention is running something like Gryphon or Asus+Merlin+scripts setup
behind a friendly but reliable dual-WAN router, like a
Peplink Balance One Core, with the Peplink set into what's called
Drop-In Mode (the Peplink would handle dual-WAN load-balance and fail-over automatically and -- this is key -- transparently pass traffic between both internet connections to router behind it without any network address translation ("
NAT") whatsoever, so you could avoid
Double-NAT, which complicates and sometimes breaks internet-related services). This type of approach would give you the best of all worlds -- scalable wifi (optionally Wifi 6 w/ Asus), some level of turn-key parental controls, and dual-WAN that actually works (and truse me, dual-WAN even on custom firmwares or certain "faux" dual-WAN products is a far cry from the likes of Peplink).
If the above approach for some reason leaves you wanting more, and you really do want to go with a community/corporate firewall, I'd urge you to take your time and plan a phased roll-out. It's going to take a fair bit of learning on your part to get it configured properly (like, probably weeks of casual learning and trial-and-error). To start with, I would leave your current home network intact, and work at setting up the firewall in a "lab" test environment, gradually getting to know the product and building out your configuration, testing as you go with a few devices acting as your kids' endpoints, your endpoints, IoT, guest access, etc. That way, you can shore up your config and be competent on the product
before you go live with it in your home network.
For the firewall platform, while Ubiquiti EdgeRouters and UniFi gateways are nice, they're not the right choice here: neither EdgeOS nor UniFiOS will have the level of access control required to give you the level of parental controls you're looknig for, nor would they be "point-and-click" (you'd have to delve into the command line). Instead, I would look at
Untangle or
pfSense pre-built appliances (an Untangle
z4 or Netgate
SG-3100), which will have the features you need and expose all of them in the web GUI; additionally, they're more well-documented and they both offer vendor-direct support that, while fairly expensive, would allow you to implement your setup the right way, the first time, should you want to pay for the privilege of skipping all (most) of the headaches. Not really the case with Ubiquiti (slow email support, user forums and a partially-completed knowledge-base), Mikrotik or any similar "cheap" multi-WAN router vendor.
Now to answer your bullet-points specifically:
- Switching on a router/firewall - You can use multiple ports for switching, certainly, but unless it comes with a hardware switch chip built-in (to offload switching at line-rate speeds -- example: Ubiquiti ER-X or most any home all-in-one router), you'll have to software-bridge the ports into the same subnet (example: Ubiquiti ER-4), which can/will throttle the overall throughput severely, and is generally not recommended. Besides, as I said earlier, if you're thinking of a discrete wired firewall, it's probably best to run a discrete core switch (ideally a managed switch) to offload the responsibility from the router/firewall entirely, and since you already have a GS716Tv3, you can make that your core switch with no additional spend (for now, anyways).
- "Power" - Yes, the firewall OS and hardware I'm thinking of will be able to handle tens of thousands of sessions and hundreds of Mb/s without breaking a sweat -- plenty of horsepower for all the gaming, streaming and general traffic flow for your entire network, without any users having any issues at any time.
- Dual-WAN - Yes, you'll be able to configure two WAN ports easily on any firewall OS / hardware I propose.
- Parental Control - This is just a friendly name for having a pre-made, automatic ability to do the things that you can accomplish with most higher-end firewalls; it's just that with the latter you have to build it out manually, as opposed to having nice pre-created templates and point-and-click shortcuts already laid out for you (which is what you get with the likes of Circle for Disney or Gryphon... except that they cannot be customized beyond a certain level). So yes, parental control is very much doable, but again, you'd have to learn how to create and use network objects, object groups, zones, rules and policies to create the equivalent "parental controls" yourself. Most SNB'ers would cry bloody murder at that, and I get why, I really do (because it shouldn't have to be that hard in 2020...) but unless Asus+Merlin+scripts or those parental products suffice, this is the kind of stuff you're simply going to have to learn in order to make the same levels of control happen for you with a product like pfSense, Untangle or a corporate product like a Fortinet FortiGate.
That was a lot to digest, I know. Take you time with your reply. Happy to help further.
. I have two modem cables so two separate internet connections and I would like to take advantage of efficient load balancing. 
