Cheat. Grab a dedicated wifi point. Something like ubiquiti AC-Lite or similar. I use it in place of my HH5 because its awful at getting through concrete walls. Also takes load off your router so it only handles traffic rather than wifi too.
The BTHub5 will only do about ~85 MBit/s without flow-offloading and around ~115 MBit/s with software flow-offloading, yes, DSA improves this slightly, but it remains in the same ballpark.
The Archer c7 is good for up to 170-200 MBit/s without flow-offloading, software flow-offloading can extend this, but it's still underpowered for this new job. Yes, the proprietary OEM firmware can make use of more hacky speedup tricks, placing this at the gate between the internet and your LAN would be a bold move though...
Both devices will still work, even though they are not fast enough to cope with your WAN speed, so it's not as if you'd be left without usable internet until you've upgraded your equipment.
Openwrt for the BT hub 5A up to and including the current 21.x stable release has a rather slow ethernet driver.
I haven't tested its performance myself (I've only used it with a 40Mbps connection), but I've read that with the original openwrt ethernet driver, the BT hub 5a can barely cope with a 80 Mbps (Megabits per second) VDSL connection. I believe I read in a iperf test it can manage just over 100Mbps.
As I mentioned above, the development branch uses a new, faster driver which is included in current snapshots, and apparently can achieve up to 330 Mbps, and that's only with the router not doing anything else.
To get better performance, you'd currently need to build it yourself with additional patches including the improved DMA pull request. See the authors posts, which I provided links to above.
My new connection will be 500mb down &60 up and apparently the ISP router is not the best.
If I continue to use the Archer C7 as a wireless AP (for now) - Is a Pi 4 the best option to handle the router part. That and a simple switch, should easily be able to handle the traffic, with SQM and even upto 1G Fibre?