It sounds about right for GH6 (6 drops is roughly 120 TDS). Those GH test kits are not the most accurate. And RO water reading of 3 is not unreasonable.
I don't know why you would have been reading 240TDS last week and now it's a 100 points lower.
I would suggest, not changing water parameter with a water change for a week, then see where it is at. Change if it gets close to 200 (say around 180-190). Let's give the tank some stability by not changing anything for a while.
I checked my TDS meter and it reads 3 on RO water and today my TDS in my tank is 133. I did a test with API and it took 6 drops so that's around 100 TDS.
I had 2 adolescents die overnight.
Should I only change water when it's over 200 TDS or should i continue with the 10% every week?
Are you sure the new reading is correct as that sounds unlikely, and I would have thought you would have had shrimps dying with that sort of decrease. Do you have a second way of checking the TDS, or can you take a sample to a fish shop for them to test? I can't think of any reason that would happen or even heard of that before? Even if you had put pure RO water in last water change it wouldn't have dropped that far!
If it is currently the TDS113 it will make adjusting easy as you just mix the water to the desired end TDS figure (140ish) and it will slowly go up to that.