Let’s do the bog-standard obvious and sane thing and pick a single point in time, once a year and use the value then. Maybe, i don’t know, close of market on the last trading day of the year. At which point it won’t fluctuate again until the new tax year. Then, we can call it “mark to market” because we’re marking the value to the market at a point in time.
Finally, we stop with silly bad faith arguments because fluctuations in stock have been successful taxed for decades. This is how day-traders pay taxes, and it’s not even a little challenging to do.