That is true. But that "good time to buy" is highly subjective and varies greatly by the individual person's wants and needs. My indicator has been RAM. Whenever the next gen iPad bumps up the RAM to the next level, that is when I buy. I've done that since day-1. (it helped me to avoid "clunkers")
That is why I ended up buying a 1st gen 12.9 Pro *AFTER* the 9.7 Pro was announced that it had only 2GB RAM. (that helped me avoid a device that was discontinued only 15 months later) And it is why I won't be looking to buy another one until the 1st gen that offers more than 4GB RAM. FOR ME, things like TrueTone and ProMotion are gimmicks that are not worth spending additional money for by upgrading more frequently.
So for me, it is RAM, for others it will be different.