You need to develop tyre fitters thumb more. Struggled with mine , 28s on giant disc rims, eventually went to lbs and mechanic showed me how to make it less of a struggle. One bead on, tube in, valve at 6 o'clock on the floor, put bead in at 12 o'clock, feed bead in and push towards floor, gives a bit of "slack" when you get to the valve, and on it pops.
Use the heel of your hand to roll the tyre bead over the rim. It has more purchase and strength than using thumbs. I know someone who dislocated a thumb using poor technique.
You need to develop tyre fitters thumb more. Struggled with mine , 28s on giant disc rims, eventually went to lbs and mechanic showed me how to make it less of a struggle. One bead on, tube in, valve at 6 o'clock on the floor, put bead in at 12 o'clock, feed bead in and push towards floor, gives a bit of "slack" when you get to the valve, and on it pops.