Thats great to hear, if all the alternative suggestions and solutions that we have all put forward have helped you to narrow down what you need to agree then thats great.
The last thing I'd advise is that you may want to consider getting a solicitor to represent you (not a joint one) and review the agreement to ensure that it is water tight with no loop holes...
This forum is littered with examples of where things have broken down, agreements not kept to leading to lots more heart ache and costs higher than what they would be to just review and arrange a consent order. Ultimately the decision is yours.
Some of the key changes of circumstances that lead to acrimony (and thus not keeping to the agreement) to be mindful are you or especially your ex finding a new partner, accident or illness, your son chooses not to see you, disputes over asset values, financial changes etc..
One other option, if you can agree between yourselves, is to use a divorce service. I've used one in the past, and on the basis that my ex and I came to an amicable agreement beforehand, they simply did all of the paperwork for us. The only problem with a solicitor is that, if you aren't careful, they will start saying that you can do better out of the divorce settlement, and if both party's solicitors are doing this, then it becomes a battle and you both end up paying more in solicitors fees than you could gain in a better settlement. A divorce service is usually a fixed cost agreed up front (usually a few hundred pounds) and generally stress free.
If you are happy about this then good fortune.
It still sounds like a recipe for disaster to me, there are so many things that could go wrong in the next 5 years to scupper this plan. I just hope I’m not reading about one of them in a few year’s time.
I hope it all goes well for you and your son.