The first BT Engineer work is done in about an hour due to all they do is install the black tubing from (in your case) the chamber to your house wall to the CSP box.
They then blow the fibre cable consisting the 4 smaller fibre strands down the black tubing, then join the chamber end to the DP and the other end gets wrapped round in the CSP tray, they do a light test at the CSP end and that's it.
About a week or two later (or maybe a day or two layer) the second BT Engineer arrives and installs the ONT and what ever else is being installed and they then drill a hole in the wall and push through the premade white fibre cable, which is cut to size and joint to the chosen fibre in the CSP tray, mine was the blue strand I think, this took just under an hour to complete.
The BT Engineer will ask you to connect up your Router (my case it was the Smart Hub) where they asked me to go to speedtest.net where they took a photo of the speed, maybe the last couple of steps was due to I got my local MP and the CEO and Chairman of BT involved.
But that was all it took.
I think you was just unlucky, we were also unlucky due to we had build issues at our Splitter Node which took a month or two to get fixed.
But if that build issue wasn't there it would of been all installed and ready to go in a week or two.
But BT can do both jobs (external and internal work) in the same or next day due to we had both work done in a couple of days.