The interstem actually sounds like the way to go. The positive benefits of MM111 rootstock but a smaller more manageable version.
As I replace trees over time, I may try going that route. Honestly, the modified central leader with a flush of small competing and similar height top limbs is fairly manageable compared to a regular pruned central leader before I made the change.
The interstems are small, only in their 2nd year. We’ll see what happens to them. I would have older ones but I and not so practiced at grafting. 
Wow! a veritible treasure map.
M111/B9-That is about the size they will be,around 7’ maybe 8’. None of mine are over 8’.
That is actually a very nice size. It gives a decent amount of fruit without being too large to need a ladder or not be able to spray the tree with a hand held sprayer. I like this size tree more than the huge ones that you cannot get to the fruit without a ladder.
Hey that’s why I watch oak island every week and drag a metal detector to the beach every year. I’m always on board for a good hunt. Sadly I only get a few nickels and drink can tabs. Hopefully this treasure map will be more giving.
I’m looking forward to seeing that apple tree in fruit. Take it from someone who has done that many times with pears and apples that some varieties are more aggressive than others. As an example, now that i know growth types, i graft slow growing pear trees like tyson to the most dominant part of the tree, the very top of a central leader. Pears like duchess d’ angoulme , carrick, pineapple are faster growers than pears like tyson. The slowest growing pears for me are comice, concorde, harvest queen etc… since you may not know the most aggressive apple up front you might have to switch some things around, prune etc. later. Those type of trees are fun and worthwhile!
Topping out around 8’ or so is what I hoped for when I started down this path. I’m just old enough to not want to constantly be on a ladder. None of my M111/[BUD9|G41] trees bloomed last year, late freeze. This year looks like many of them will. Oldest ones grafted and planted in 2020. No staking/support for any other than the occasional piece of bamboo to straighten out a leaner. Anyway, fingers crossed.
Thanks.
Wow, you have learned a lot of nuances about grafting that I had never considered. Thanks for the encouragement!