Don't get in a tizz! You can subscribe to InDesign CC2015, which runs under El Capitan and subscribe by the month, so you only pay for InDesign when you need it. That's just £17.15 in the UK for example - that won't break the bank and you have access to all the new InDesign goodies!
I have investigated that and it is not really useful. It means that every update I would incidentally do would cost me a month's subscription. If it was really pay per use (per day) I could consider this because it is low risk. But not per month. I use InDesign very sparingly, but I use Acrobat Pro regularly.
I might do CC, I can even get a good deal for the coming year (at ±2 months fee), but the problem lies afterwards. Then my document is in a version that cannot be opened in CS6 (for which I have bought a perpetual license in 2012). I'm then bound to CC and not free to do what I want. If I don't pay, I cannot even open my own documents. That I find unpalatable. I don't like the risk profile. I've been thinking about using CC and at the end of the project save it back to CS6. But I worry if that will be relatively painless.
Anyway, I can just keep on using CS6 and maybe someone can answer the actual question I asked instead of telling me to go to CC. Just assume that I am not going to CC, what are my options with CS6 on the Mac?
from my personal experience, InDesign CS6 v8.1.0 is running well on OSX 10.10.
From a lot of reports I've seen here and elsewhere OSX 10.11 ( ElCapitan ) is not the way to go.
Hope, that helps.
Bottom line: If you move to El Capitan, be prepared for a world of pain with CS6.
You can’t upgrade an operating system in a vacuum; especially on Mac where Apple doesn’t give a rat’s behind about backward compatibility.
Sticking to old systems and apps, TOGETHER, is wise. Beware of this: new hardware will get you too. New Macs can only ever run the Mac OS newest at their release. High resolution monitors need software support in Adobe apps. So bear in mind you need to stick to old systems, apps, computers and other hardware. As computers fail you may need to buy an antique.