Hell of a post Ben, so thank you for all of that information.
Last couple of years, I have done about 1000KM (600 odd) per year, and that was without properly trying and also fitting this in around family, my allotment etc. Also, I was cycling to work when possible to get the distance in. Working from home now so, I plan to got out 2 times per week on my lunch, about 20-30KM/45-60mins and that seems to be a decent distance for a lunch break.
Normally I am an early bird, and would happily wake up at 5am and ride for 1 hour before everyone wakes up. Problem is that everyone is awake at 4am so I am on Dad duties.
Weekends are just like you, with both sides of family being big, and with young kids it is just going for a ride when they are asleep, normally 30-50KM.
I have two bikes, one for family rides and one for rides. I am going to get out in the morning to get my son, and use that as a "commute" cycling to the office and back about 10KM per day, and where possible fit in a big lunch ride.
If I am to do the above, there is no reason with commutes on top that I could not push 100KM per week, but I am also being realistic and setting myself a target of 30KM per week for this year, roughly what I have done last year and anything on top is bonus.
Winter time is probably where I have not done well, I have a basic turbo, but did not get it out as the twins were due and space etc. But will probably get into this more when I have a better routine for the family. I do want to stress that I am only 2 years into really properly cycling, and have only get a proper road bike this year. Previously, i have had a 14 speed city bike which was 10 years old. This is my "thing" now.