I've done load dev with and without a chronograph. I've gotten good results both ways.
Having velocity data (especially extreme spread data) makes it easier I think, especially if shooting at long distances. Knowing my ES helps me know how much vertical dispersion I can expect to have on target, no matter how well I shoot the gun.
It's also helpful when initially conducting a ladder test (I do 1 shot per powder increment), to find the ballpark I want to be in, then refine from there. (Don't tell the guys at Hornady)
Once I'm finalizing a load, an ES of 30fps isn't awful, but I'm looking/ shooting for 20fps or less.
And yes, the Garmin is a game changer. As has been stated by others, if you've never owned a chronograph before, you shouldn't be able to just buy a Garmin. You should have to suffer with the old types like the rest of us did!