That was fun (early morning, July 25, 2021)...
I set up my 8" Newtonian last night to observe the Ganymede and Europa shadow transits. I started at approximately 0730 UT when Ganymede's shadow was approximately 2/3 across the disc. I didn't see Ganymede. The seeing ranged from poor to good. While I never lost sight of the equatorial belts (seeing wasn't that bad), sometimes the shadow disappeared. Other times, though, seeing snapped-to and I could see details in the belts. I used magnifications of 167x, 200x, and 250x. 200x was the sweet spot when seeing popped, 250x was almost always too much.
I missed 1st and 2nd contacts for Europa's shadow, but I came back outside just in time to see 2nd contact for Europa and 4th contact for Ganymede. What a great coincidence the moons left and arrived at so close to the same time. For a few minutes Jupiter looked like it had ears. I was able to follow Europa for a while as it transited, but I lost it after a while. Another pleasant surprise was to see Europa's shadow and Europa flanking the Great Red Spot.
Interesting phenomenon I noticed last night: when Ganymede and Europa were close to Jupiter's limb, each moon looked quite bright and very white. Previously, I was unable to find Ganymede and later I lost Europa. As the satellites get closer to the central meridian, do they get lost in the glare? Does that also cause the contrast between the planet and the moons' colors to decrease, making them harder to see when transiting?