I generally think once you get below like $500,000 on bonuses you really are now into low probability and bonus $ and player talent are not correlated, it is more about signability and leverage. In big money pitching signings I think you are generally paying for now stuff, and that doesn't guarantee future growth, especially when you are running big reliever risk because you are paying on things like velocity and not really mature pitch mixes. It also isn't necessarily a strategy that is playing out across orgs, there are not a lot of J2 pitchers on top prospect lists and there are not a bunch of big money guys out there either.
Generally with pitching you should sign it when you see it. You can't churn so be vigilant on older players and make the signing. I would rather be scouting 16-18 year olds than 14-15 year olds when it comes to pitchers. That means less money spent, but not less talent. It also isn't less money as a plan, it just is the market forces. In the same way that in the draft you only can sign so many high school pitchers, but you are bringing in a lot more college arms and especially senior signs at the end.
My overall thesis is that using bonus as a measure of talent acquisition is generally a flawed piece of analysis in a space where market forces drive certain outcomes. Looking at actual player outcomes, especially as they arrive stateside is going to be much more telling.
Now if you want do your own digging, I tracked a bunch of DSL games I got access to and recorded the data I could. It is easier to find on the 2022 tab here, but almost all DSL pitchers should be present.