My thinking is this.So should I wait until there's 9k on the clock before booking in for an oil service, rather than having one at the 12 month mark?
The VW spec oil is long life for the extended interval, the risk is if contamination builds up. In the modern transporter engine the main contamination is likely to be diesel due to the way DPF regen is done, especially if the regen is often interrupted.
I know I'm mostly doing long runs, I use the start/stop to alert me if a regen is running at journey end and I keep a broad eye on it via the VAG DPF app.
So personally I'm happy having it done once a year, coincidentally it was just over 9k last time but it may be a bit more this year. I'd probably not let it go over 12k
If you're not keeping an eye on regens or doing lots of short runs where they may be interpreted then maybe aim to change earlier to offset the risk.