Let's be honest. 'Voluntarily' is never coming into it.It's your timeline so do what you like, but I don't see that as being a practical possibility for long. An ongoing, indefinite transfer of that magnitude is unprecedented and would run into exactly the transfer problem of WW1 reparations, while being higher than post WW1 reparations (as a % of GDP). As for the argument that German spent that much on defence so it is affordable, that ignores the nationalism issue and that spending money inside a country is fundamentally different from giving it to foreigners (money given to Germans leads to higher spending in the country and so higher incomes and tax revenue, money given to foreigners doesn't, or at least much less so). This is why Britain could repay its domestic war debts but not its much lower foreign debt, and Germany struggled to pay reparations despite the debt burden being much less than France or the UKs domestic war debt.
Aside from that I don't see anyone agreeing to it voluntarily for long after the war: if it is to suppress the Germans then the Germans won't agree to it and it seems unnecessarily large. If it is to defend the Germans then the Germans would prefer to do it themselves and it's unlikely the British or French would actually be willing to shed much blood in that cause, meaning it lacks credibility, which is critical for deterrence.
As I say though, it is your timeline and I look forward to seeing what you come up with.