I'd personally hope they just force open sourcing their firmwares if they want to stay in the market. I really like my Omada stuff, ubiquiti is just a tough pill to swallow on price.
Signed firmware just means you can prove a given key was used to sign something. Most Linux distributions sign their packages so you know one of the trusted keys from the maintainers was used to sign the packages (and yes, this includes firmware), which prevents a man-in-the-middle from modifying packages.
The only problem I have with signed firmware is if there's no way to change the acceptable keys. Signing itself is an important security feature, its only problematic if the user can't upload their own signed packages.
I recently bought their Flint 2 (GL-MT6000) based on multiple recommendations online when looking for a router that supports OpenWRT. That's preinstalled, with AdGuard Home and WireGuard VPN on top of it. I'm looking forward to set it up and play around with it.
What do you exactly mean when you describe their approach in software as Android-like? That it's easy to install services in OpenWRT?