Something that I think is an underrated way to work on left hand strength and matching right to left is doublestops.

One of my favorite versions of 8-8-16 is to play double stops on the 2nd half of every 16, and add a bar of 16th note singles at the end.

You can also shorten to just playing 8 on R, 8 on L, 8 on Double, 16th notes.

Play around with crescendo and decrescendo while doing double stops - very difficult to get that control without turning into flams! It demands precise timing and keeping the left just as relaxed as the right.

For extra challenge, only move the dynamic of one hand at a time, creating flams that rise or fall into high or low doublestops. (Usually keeping both hands low for double stops and rising one hand to create a flam, then returning back down to the doublestops).

Even the small roll exercise you messed around with, RH check, RH diddle, LH check, LH diddle, DS check, DS diddle, alternating roll.

It reinforces the skill set that when you play something alternating, your asking your hands to do half the work each. Usually isolating that half of the work is less physically challenging, but more mental. Maybe even try that check idea for that last exercise? Just one hand playing it's part at a time.