testing for TDS in hydronic systems

Great questions.
Pretty much any chemical treatment you add to DI water will change the TDS. But the change, increase, is for the better when properly dosed. Really TDS is a measure of the conductivity of the fluid, anything added will increase conductivity, salts, dirt, chemicals, etc.

You will need to use the test kit that Fernox supplies to check and confirm the system fluid quality now.

I feel the treatments add enough value for the $$.

Ph buffer, oxygen scavanger, and most critical for brand new boilers and piping and components is a film provider component.

The micron thickness film protects all the bare metals, reduces scaling potential and protects against aggressive fluid attack, should the conditioner ever get depleted.

Know with chemical treatments that they need to be maintained, check every few years and possibly boosted from time to time. The test kit confirms the quality of the fluid.

Still others feel conditioners should not be required with demineralized fluid in a cleaned, sealed, leak free system that never takes on additional water.

I'm seeing some of the new boilers are shipping with a container of inhibitor in the box, and/ or recommendations for brands or types they approve.

Warranty of that investment is tied to the quality of the fluid inside, you made a good choice.