Is your tree mature with a full crop? Like several other E. plums I grow, the ripening period is relatively widespread. I will harvest my Valors over the next month. One I picked today was at 19 but I am confident they will begin rising into the 20’s.
Incidentally, my version of Valor came from Treetop Nursery first and Adams second. 5 years ago, or so, Adams switched varieties. Pretty much seals it that the original was also the genuine.
The large plum tree I lost was probably on myrobalan seedling. I thought it was just as water tolerant as the other plum rootstocks I have. I’m just as surprised by what wasn’t affected as by what was.
My tree is on Marianna 2624. I’ve bent branches so it does not grow very tall. It is about 6-7’ tall. Planted in 2014. Set fruit the first time in 2017 and set well from the beginning.
This is the 3rd year of fruiting, loaded. I need to use all kinds of support to keep fruit off the ground.
That’s elaborate protection.bb
Desperate people use desperate measures 
Besides using the frame to tie ropes on to keep branches off the ground, nothing else there can protect against any pests.
So you’ve got a naturally dwarfing tree on dwarfing rootstock. Maybe your high brix readings are partially the result of such rootstocks. I’ve never compared fruit on different rootstocks because I generally need vigorous trees to grow above the browse line of deer.
It is common in the literature to suggest that dwarf apple trees produce higher quality fruit- I always assumed this was because they don’t require such skillful pruning to realize the best fruit.
I have an ax (or rather, pruning saw) to grind on the subject. However, in the humid region it’s easy to see how less vigor could mean less access to water.
My former helper for a decade (he now owns his own business fashioned after mine) worked at a commercial orchard for 10 years. The owner had a great side business of selling trees to landscapers to plant for people like my customers. One year someone used a tree-spade to dig up a Fuji apple but ended up dropping it back into the ground in the same spot after severing much of its root system.
That tree produced, by far, the best apples of any tree in that row that year according to my former helper.
