Spoiler, its RDT
In case people do nto know what RDT is, which they really should if they have been into coffee for a little while as it makes a big difference:
RDT is Ross Droplet Technique, which is very much adding water to beans. Named after David Ross who came up with it back in 2005
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Not sure if you have watched Lance’s video I linked elsewhere in the post but they measured particle size, RDT improved uniformity of particle size. This to me is the first empirical evidence of the actual benefit of RDT over and above less mess with grinding. For what can be a completely free and quick upgrade that seems always worth doing.
While WDT does need a tool and even a homemade one isn’t completely free it’s ability to better distribute grinds in the basket I would also say it’s an essential upgrade as it can be so low cost
Anything else like slow feeding, hot starting that are free upgrades no matter how small for cheap to midrange grinders that lack prefeeding augurs or other chokes that prevent overloading the burrs seem no brainers to me.