I don't see anything troubling on your TBB Ping Monitor. The very brief outages all occur at night and could easily relate to BT network maintenance.
Earlier in the year I was asked to look at the reliability of a G.Fast connection in London with a BT Fibre 250 service because there were lots of connectivity problems at the site.
I installed a Fingbox which performed six automated speed tests every day and gave continuous connectivity monitoring. I could also monitor it remotely.
For the whole of July the results were;
- 263.4Mbps average download
- 49.2Mbps average upload
- 2 outages of one minute each (>99.995% uptime)
I wouldn't call that unreliable.
The frequent connectivity problems turned out to be related to the DHCP server on the Smart Hub 2 and have been resolved by putting in a firewall which acts as a DHCP server, among other functions. There haven't been any significant outages since putting the firewall in.
That's
my real world firsthand experience of testing G.Fast for reliability and performance. Unbdoubtedly there will be others with much more experience and some with zero.