I cannot believe the mileage you are all getting.
Costly business cycling in Ireland! The longest I have ever gotten from a bottom bracket is 10,000km. That’s press fit or Shimano Hollowtech ll.
Headsets need servicing regularly, but depends largely on the bike.
my current bike, 2nd hand, i already logged ~22k km (first owner may have logged some unknown kms) and sent to the shop for service before but not sure if they serviced the BB tho…
like you said - you mostly ride in wet weather.
I rotate 3 chains every 2000km. The reason is that if you wait 6k, the cassette already has considerable wear. By rotating 3 chains every 2k, the cassette is almost as good as new after 6k.
And I continue with that set until I experience the first shifting problems or chain jumps. Then I replace everything, new set of 3 chains and a cassette or the 5-6 middle cogs because the smallest and largest ones are mostly still good. Depends if separate cogs are more or less expensive then a complete cassette. I easily do 15 to 20k with a set.
But I wax, cold wax in my case and that can make a huge difference when regularly riding in wet weather. I live in Belgium, not as bad as in Ireland, but surely not as dry as South-Africa.
Waxing greatly minimizes sticking grid, dust and sand. Those are the particles that cause wear. Starting out with waxing is quit a bit of work because it is absolutely necessary to thoroughly degrease all drive train components. Once you’re started it’s a breeze. Things remain much cleaner and simply cleaning with a dry towel removes 99% of all debris. The thing is that you have to remember to clean and re-wax after your ride. You can’t quickly wax before going for a ride because it needs to dry. Re-wax every 300-400km or after a wet ride.
To degrease I found this method the least labor-intensive and with extremely good resuts:
The dishing washer soap is a very good degreaser. The lightly boiling water makes the air bubbles and the water move around and acts very similar to an ultrasonic cleaner. Things come out shining like new and the best part is that your hands remain clean 
The first wax after this treatment must be a double pass. Let the first pass dry and add a second pass. When I swap chains at 2k, I visually check the drive train and if necessary, I’ll do a clean like above. The frequency of those thorough cleanings in my case is 2 per season.
Hot wax is probably even better but I consider that a bit too much work. I assure you that riding with a waxed chain will make you wonder why you ever used regular lube.
I use Molten Speed Wax (new formula) and feel that it is no more difficult and similar to your method. The only difference is there is no need to rinse since after the first cleaning since the chain is fully immersed in 200° wax every 150 miles or rain ride.
I live at the beach (salt air) and get 5,000+ miles using YBN chains.
Love all your kilometer munching and look at it with my MTB envy, but lets get back to topic 