Yes, and it can be overridden to all client devices connected to your main router.

I think Brume2 is better suited to adding VPN and other features to your home network without replacing your main router. It has 8GB of storage space so you can also install more third party plugins. Flint, on the other hand, is better suited as your main router and it doesn’t require Drop-in Gateway mode.

Which one is better depends on your actual needs. There is no difference in VPN and AdGuard Home functionality between the two methods.
Brume2 as a Drop-in Gateway will increase latency slightly and is not recommended for gamers.
Flint does not currently have Mesh (seamless roaming) and is not specifically adapted for IPTV. if you don’t need them, you may consider using it as your main router.