Shakes as it grabs. As you are slowly coming off clutch
Sounds like warping of a mass plate, or a bent spline shaft, possibly even vacuum booster. Weird issues sometimes present when we change things.
First gear I would bet you use more torque on average which is why it doesn't present in reverse.
Could be the extent of the torque on the bushings.
What you describe, shakes as it grabs slowly coming off of the clutch also seems like there's something causing lubrication between the pressure plate and the flywheel, brake cleaner can clean it out and evaporate from the bell housing.
Enough torque can crumble a flywheel if it's defective, however, unlikely in 30k
I could see the slave being loose causing this.
Whole front end has been rebuilt in last 10k miles
Whole front end? steering rack and bushings, a-frame bushings, and the tie rods and ball joints? what parts were replaced exactly during this rebuild?
Another thing that likes to present at this torque level are loose drive axle bolts that hold the CV to the transmission, so check to make sure they're properly torqued with applicable tripple square, or torx bit depending on the heads.
I tend to see first gear issues as a high torque issue and one test for this is to put it in first and try to make the same thing happen when not releasing the clutch. In a parking lot let out slowly on the clutch without pressing the accelerator, then accelerate really aggressively and take your foot off the pedal without depressing the clutch and let it slow to idle speed again. This is testing for high torque issues that have nothing to do with the clutch system. If you can get it to shake when not using the clutch then you need to look more closely at the engine and dogbone mounts and cv axle's. If you can only make it happen with the clutch then it is specifically related to the clutch and components of the clutch.
One quick thing to check is that the slave cylinder isn't leaking inside the bell housing causing it to slip, especially if you have added any brake fluid recently.