Welcome Alan, I'm in a cold climate zone 4 and I have to create places to protect zone 5 trees/fruits. I have a lot of land and am lucky to have it be 2/3 to 3/4 forest. I am trying to plant forest farm strips on my swales, but so far I've just killed a lot of fruittrees any suggestions on species or conditions I need to create to be successful?
There are some things you can do to create a favourable microclimate, such as planting on south facing slopes or next to rocks and bodies of water for frost sensitive plants. But it sounds like you are doing these things already. Are you sure you have exhausted all the possibilities of your hardier trees before putting so much effort into planting things that don't grow there naturally? For me New Hampshire is in the far south, so everything in my book should grow well for you, and more besides. Remember that trees aren't just about fruit, but include nuts like hazels and butternuts, salads like lindens, shoots like harigiri, koshiabura and toon, spices like Sechuan pepper and saps like sugar maple - and that a forest garden isn't just the trees. I get over a hundred species from my small garden without doing anything especially fancy for tender plants.
I've tried apples and even when they are zone 4 they don't make it in the swale. I'm going to plant brambles to nurse along some butternut and around the fruit tree. It'll block some wind and hopefully deter deer til they can get a little bigger. I'm going to try some rocks/stone piled up behind any zone 5 plants. I already have significant sugar maple and some Linden trees and use them, but not every year.
I have never heard of the other three plants and I'd like to know more about the Sechuan pepper. Is it cold hardy?
There are quite a few nuts trees that will grow in your area, walnut, butternut, buartnut, hazelnut, hickory and chestnut.
There is also pears and plums, sand cherry, and other native fruits from the prunus sub-family.
There are alot of berries too (raspberry, blackberry, strawberry, currant, gooseberry, jostaberry, atonia, elderberry, etc)
In general I don't like planting apples trees or others from the rose family, that comes from Europe and isn't native to the Americas, they tend to need alot of spraying and such. That said there are quite a few zone4 apples. Have you gotten a chance to check out the regional fedco nursery? They have a nice selection. https://www.fedcoseeds.com/trees/apples.
If the apple trees are being killed by deer/etc, you might need to fence them in right under the eaves of the house (I am only half kidding/lol).
They do have plants at reasonable prices, but not much by way of fruit. They are very against invasive species here and still backward about gooseberries, and currants even though we don't have much of a white pine industry here.