I'll only answer this part first:
is it impractical to expect a seamless connection from a load-balancer? (E.g. No dropped frames in a video call even when one of the ISPs goes off). If so, is there a better alternative?
With a basic "load balancer" this is very difficult to achieve as far as I know, simply because each of the 3 ISPs requires you to use a different IP address to access the Internet.1
So whenever the link through ISP 1 goes down, all TCP connections made using that ISP's IP address are as good as gone – there is no way to migrate them to a different endpoint at all, much less without losing any packets. The same also applies to most UDP-based applications (there are a few which support endpoint roaming, but most video streaming protocols do not).
It's quite possible (though I have no way to confirm) that it is actually an unexpected ISP switch that causes all existing connections to die. You should watch your router's system logs or check what is being reported as your "public" IP address from the affected computer.
One possible workaround is to set up a VPN that uses a roaming-capable protocol (WireGuard in particular works well), and always make the video calls through that VPN. This works because your video connection will be using the VPN server's IP address the whole time, regardless of which ISP you're using to reach that server.
(But even with a roaming-capable protocol, there can still be some packet loss – if the link to the current ISP dies (i.e. switchover is unexpected), the server will not be aware of what just happened on your end and will just continue sending packets to your old address until it gets informed of the new endpoint. The protocol needs to be designed to buffer and retransmit if necessary.)
1 (Large enterprise networks use BGP to share the same IP addresses across multiple ISP connections, but this can only be done with the ISPs' cooperation and has quite a few additional costs.)
In inspect element mode of the browser and while you are on the password field change
input type = passwordto
input type = text:), man who would've thought hacking WiFi would be that easy through the browser.