An Emergency department perspective:

We have more falsely low O2 readings causing ED visits than real ones. If someone is having difficulty breathing, it doesn’t matter what their O2 sat is - they may be compensating or not - either way, we need to see them. If their breathing is fine, then a low O2 reading is often spurious as some degree of training and discretion must be used in interpreting results and getting an accurate reading.

BP is another issue. While it is good to have a home BP cuff and they do tend to be accurate, most patients are ill-informed regarding what constitutes an emergency; however, ED visits for a higher than usual BP must be handled carefully as patients paradoxically do not appreciate being told that they are fine and do not have an emergency once they have opted into an ED visit.

As for glucose monitors, there is essentially no role for them in non-diabetics from a medical perspective. For OCD data collectors, they are probably wonderful for micro-managing I am not sure what.

and go to the ER if my oxygen went below 89, even if I were feeling fine, breathing normally.

It will pass (pun intended). Just like other food fads.

but how does it react to YOU. Does cooling the rice really blunt the post meal spike IN YOU?
We are all just a bit different.
As for the BP machines it's simple to just bring the BP machine with you to the doctor's office. Compare the doctors readings with the readings on your machine and adjust accordingly..
Mine is bang on when done properly. Some med assistants/nurses don't conform with the
AMA procedure to take a correct measurement
https://www.heart.org/-/media/files/health-topics/high-blood-pressure/tylenol-hbp/aha_toolkit_poster_final_102618.pdf
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