Is there a way to disable ECMP?
Yes I am aware I can change costs but its just yet another administrative overhead, i'd rather just disable ECMP as we don't need it anywhere in our network

If you don't care what path will be chosen between two points (which is the case, as you don't want to fine-tune the path costs), then why do you care if it is ECMP or not?

Vast majority of our environment is wireless. Link speeds are inherently unreliable as they change with conditions. Trees grow, buildings get constructed in the path, others put up radio's that cause interference etc etc etc there's a million possibilities that ultimately mean you cannot predict anything for the future. I could go ahead and test every link and fine tune path costs right here and now, and that could be completely pointless in 6 months time

As a result, vast majority stay at default but do get adjusted when necessary, but point to D to point G might end up matching up costs through 2 different paths
I don't care if it initially takes a bad path. What I do care about is completely unpredictable routing and even more head scratching whilst trying to troubleshoot and find the problem. If it consistency takes a bad path thats preferable to sometimes good sometimes bad as its easily identified with a single traceroute and tests between nodes

The last thing I want is 2 separate paths being used. Plus see above about unpredictable conditions. Consistently poor is better than intermittently poor and this can be identified and fixed much faster

Even if you somehow disable ECMP, you will still have "unpredictable routing".
Only instead of having ECMP routes, which at least are easily seen in the routing table, you can end up with asymmetric routes.

We can sit here and debate this but here's the simple reality: ECMP does nothing beneficial for us and only introduces even more unknowns
My goal is not to say "well we can't be 100% totally guaranteed of everything so lets just give up, close the whole operation down boys" it's to eliminate this 1 other variable that never benefits us

So can we turn it off?

You cannot turn it off.
And even if you could in your scenario it will introduce other mentioned problems, like asymmetric routing unpredictably. Only right way that fits your described scenario is to adjust interface costs.

Ok it can't be turned off, fair enough
But it isn't a bad idea to turn it off, many other vendors allow you to turn it off for precisely the reasons i'm mentioning, it just adds another variable to troubleshooting
Asymmetrical routing is not a good excuse, it's already asymmetrical with ECMP anyway

I am not saying ECMP is not asymmetric. Turning off ECMP might be useful but not in the case you are referring. You will not get consistency you are asking for.