Is upgrading my grinder actually worth it?
It is worth it if your current grinder cannot hold a setting, cannot go fine enough, or has visibly worn burrs. It is mostly not worth it if you already own a competent 54 to 64 mm burr set and you are chasing a spec sheet.
Be specific about what an upgrade buys. There are four real improvements available: a narrower particle distribution from better burr geometry, more repeatable adjustment, better alignment so the burrs perform to their potential, and lower retention. Everything else on the marketing page, motor RPM, coatings, app connectivity, is secondary to those four.
The upgrade is clearly worth it when you have a symptom, not an itch. If the same setting gives you a 20 second shot one morning and a 40 second shot the next, the grinder is not holding its gap and you cannot dial in on top of that. If you are bottomed out at zero and shots still gush, you have run out of range. If your grounds look visibly mixed, dust plus visible boulders, the burrs are worn or misaligned. Those are all hardware problems that no technique fixes.
The upgrade is usually not worth it when you are moving between two competent grinders in the same class. Going from one well-built 64 mm flat to another well-built 64 mm flat mostly changes the flavor profile rather than the quality level. That can be a legitimate choice if you want more clarity or more body, but call it what it is: a preference purchase, not an improvement.
Here is the ranking that we would defend. If you own a decent grinder and a mediocre setup elsewhere, a precision basket, a proper scale and a WDT tool together cost a fraction of a grinder upgrade and will change more shots. If you own a good machine and a genuinely limited grinder, the grinder is the ceiling and everything you spend elsewhere is wasted. The question is not whether grinders matter, it is whether yours is currently the binding constraint.
One honest note on diminishing returns. The step from a cheap conical to a well-made 54 or 64 mm flat is large and obvious. The step from that to a flagship with premium burrs is real but small, and it will be invisible in milk drinks and mostly visible in light-roast straight shots. Spend accordingly, and be suspicious of your own excitement.