Gidday Scooter and welcome to the forum with a very well written post that also has great clarity.
The advice Guest 7403 gave is very sound. I agree that what your ex is doing amounts to cohersive family control, classed as family violence.
I can't give you expert advice at all, but will happily pass on my thoughts based on the arrangement I made with my ex over custody when we separated.
Don't stand for this crazy timing that suits only him anymore!
It may take a little effort finding a lawyer who wants to help and understand your issue without stringing things out with expensive letters. Ask around your friends and at work about family lawyers, there might even be a Pro Bono lawyer you can chat with.
Keep a diary and very good records of what your ex is doing - start that today. This is very good evidence the lawyer will want to see. Make sure he uses it! When you find a lawyer you are comfy with, ask for a short consultation and be clear with your objectives.
They could be along the lines of:
1. I suggest setting an agreed custody schedule in place eg every second weekend or similar when your ex can see the kids. The custody schedule should state pick up, drop off times and place - only able to be varied with your permission.
2. Forbidding him to make other arrangements for the kids unless through you first.
3. Lastly, forbidding any contact of any sort from your ex (AVO?) unless picking up or dropping kids off.
I assume financial and assets settlement has been agreed?
Advise the lawyer right up front what you objective are (eg see above), and ask for an estimate of all costs.
It could be that a letter may be all that's required - but given your ex's behaviour you may have to get a court order registered.
Be brave and settle this. You sound like a great mum and deserve some clarity about the future.
Hope this has helped a little - happy to discuss at any time.
All the very best! The Bro