One advantage is OpenWRT versus iOS/Android. User utltimately has no control over the later, whereas user can easily compile OpenWRT from source.
For example, OpenWRT allows more control over DNS settings. There is no user access to /etc/hosts or /etc/resolv.conf on iOS/Android. User can easily run servers on the e750, e.g., DNS or proxies, and inspect the contents of traffic, including TLS. The e750 has fewer limitations when running servers on Android with something like Termux. Using Termux, it is difficult if not impossible to run essential programs like tcpdump. Moreover Termux is not available for iOS. iOS is intentionally "locked down" and Apple prevents users from compiling iOS from source.
The e750 can accept CAT5/CAT6 Ethernet cables. Wired internet is an option. "Smartphones" are wireless-only.
I'll point out that rooted Android is an option that grants you much more control. You can even build Android for many devices from source if you're so inclined (though perhaps not all of the drivers).
Rooted Android increasingly tends to be an option best reserved for technically-sophisticated users, but compiling OpenWRT from source isn't exactly consumer-friendly.
What are the resource requirements for building Android from source versus building OpenWRT. The option that requires much more significant resources is less consumer-friendly, IMO. It is certainly less friendly for this user.
OpenWRT is a smaller OS than Android and less complex.
For me, OpenWRT easier to work with than Android. Perhaps I am not "technically-sophisticated". I just want more control. I prefer commandline programs to "smartphone apps".
I am not suggesting a specific course of action to you; if you're happy with the approach you're using, I don't think I have a better one for you.
Instead, I am offering corrections as you requested. It is possible to manipulate the hosts file and low-level DNS configuration on Android with root. It is possible to use tcpdump on Android with root. I expect it's reasonably possible to run servers on Android with root, but there seems to be less interest in that (a long-running server is surely bad for the battery life of a typical Android device, for example).
iPhones/iOS leak stuff past the VPN constantly. I am also keeping my location changes private from Apple, and the phone maintains a persistent connection to APNS at all times, leaking the client IP, and will do so even if the VPN on device goes down. There are also DNS leaks exposing the device's client IP.
I also have root on the hotspot device and can block access to specific IPs or hostnames, and can run tcpdump to monitor traffic. It's pretty nice for seeing what spyware various mobile apps have embedded in them.
You could also run the VPN on the phone itself, and just use the firewall on the hotspot to prevent traffic to any IP other than the VPN endpoint, closing the iOS VPN leaks, but I connect 3-4 devices to the hotspot and want VPN on all of them, so doing the VPN on the hotspot is slightly more convenient.
For example, OpenWRT allows more control over DNS settings. There is no user access to /etc/hosts or /etc/resolv.conf on iOS/Android. User can easily run servers on the e750, e.g., DNS or proxies, and inspect the contents of traffic, including TLS. The e750 has fewer limitations when running servers on Android with something like Termux. Using Termux, it is difficult if not impossible to run essential programs like tcpdump. Moreover Termux is not available for iOS. iOS is intentionally "locked down" and Apple prevents users from compiling iOS from source.
The e750 can accept CAT5/CAT6 Ethernet cables. Wired internet is an option. "Smartphones" are wireless-only.
Corrections welcome.