I currently have FTTC, using an RT-AC68U router, connected to the HG612 modem. I don't believe this router supports G.Fast so would I be thinking this is currently one of the better routes to purchase that supports G.Fast, Asus DSL-AC88U ?
The HG612 is the modem, and doesn't work with G.Fast.
You may be able to blag a Huawei MT992 G.Fast modem from OpenReach though they aren't meant to give these to BT customers.
The RT-AC68U is the router, and will work with G.Fast provided you use a G.Fast modem.
An Asus DSL-AC88U is a router with a G.Fast modem built in.
BT will send you a new Smart Hub (modem/router in a single box) that would result in things like double NAT if used with your own router.
You have 3 main options.
1. Obtain a G.Fast modem to use with your RT-AC68U.
2. Use the supplied BT Hub.
3. Buy your own modem/router (like the DSL-AC88U)
Personally I wouldn't touch the MT992.
I like seeing my line stats and the MT992 is completely locked down with zero info.
This may not bother you though.
I wouldn't touch the Asus DSL-AC88U either.
I love the Asus RT range (I use an RT-AC68U also) but their DSL range with built in modems don't have such a good reputation.
At this moment in time G.Fast modems are all £150+ and are few and far between.
Grab a modem from the OpenReach engineer if you can and you your current Asus router.
Failing that use the BT Hub.
I don't recommend dropping £200+ on the Asus DSL-AC88U or £150 on a G.Fast modem as these prices will drop as more devices hit the market.