When I moved here, half the ground had no living cover, and many of the shrubberies were dead. Last summer there was extreme drought, and I didn't do much because I broke my leg. This summer I planted lots of variety, watered, limed and fertilized. I also added 1 beehive. I added last year's compost (7 m^3) in hills.
Current conditions: pH is generally 6.5, low nitrogen, high P and K
I intend to keep adding nitrogen fertilizer to fix the low test result. I hope to add horse manure later.
Cover crop observations:
I added Buckwheat heavily at first because little else would germinate. Now I don't use much of it because it's bad at surviving. It is very heat and drought tolerant when it's under 3 centimeters. Clovers do well, but won't get tall. Winter rye and hairy vetch are the best, but they both die in the summer. Rye is extremely intolerant of the summer. It needs to be consistently below 75 Fahrenheit to establish rye. Most tall plants die in the summer. I probably should have used Sudan grass.
Green beans die suddenly, but are the biggest crop. Tomatoes take no effort. They are the second biggest crop, and I didn't intentionally plant them. I don't even eat fresh tomatoes. Potatoes do well. The asparagus that survived my planting mistakes did well.
Cover crop goals:
The soil repels water a bit, so I would like to till more. I would like perennials that till like radish.
I am going to try to make a cogent reply, but I have to admit I am not totally understanding your goals. Specifically are you looking for a garden or grass or forest.
That would be my first decision point. I know Lowell and was born near there, you will observe it is well-forested at maturity. The whole place wants to be hardwood forest, and will return if you let it. Someone made a savanna and that is not an efficient ecosystem in Massachusetts. If you want a forest, just plant the trees you want at maturity. Treat mineral deficiencies and top dress some compost and call it done. You have a great amount of compost! Wish I had that much!
If you want grass you need bacterial culture. Putting down fertilizer will give you a nice green grassy field as long as you keep putting down chemicals. You will never get soil, you won't develop the soil life. And you will have japanese beetles to beat the band. Your impulse seems generally correct, you want drillers and biomass producers and then your grass and supporting nitrogen fixers will have some texture and improved infiltration/retention in deeper levels, but without soil health - from adding nitrates/urea/ammonium you won't have nodulation in your n-fixers and the microweb will be stunted- and each year will be a repeat of the last one. It sucks your buckwheat has not been impressive, I suspect once you have some better texture it will be able to get some roots down and might be worth a repeat try. I would recommend some perennial taproot plants like chicory, alfalfa and sweetclover, they break the job up over years, while buckwheat just can't get deep enough in its short life. Both of those can probably be found in seed around now where you are, or you can buy seed. I am a fan of fall planting but you are a good deal colder and it may be better to early spring plant.
If you want garden soil and have hardpan, the easiest thing to do is get woodchips and just lay them deep and plant next spring, call a bunch of arborists and pay them a little for clean loads (especially with leaves in it right now- cheap nitrogen). I had rock hard soil and it really only took about six months to get way better. Potatoes and tomatoes are loving it. No added nitrogen at all. If you want grass in a few years you can just quit adding chips and use the rich dirt. I planted some radishes and buckwheat in the chips (accidentally) and they are the biggest on the property!
Standing on the shoulders of giants. Giants with dirt under their nails
I apply fertilizer at 1/4 or less of the recommended dose near the end of every intense watering. It's effective. I've seen it. Fertilizer is temporary. I would like to add wood chips without smothering too many plants. It would be nice to have tall plants growing over or into the chips. There are limitations to chip adding. It could create steep slopes or direct water towards the house. I was thinking of putting potatoes on the ground and piling chips over.
The compost was from leaves. I also have to deal with slope issues near the house. This will either involve removing dirt or building a retaining wall. I would like to add lots of wood chips everywhere, but I will need advice on dealing with the resulting slopes (wall, or dig?).
Trees don't grow well. I think they grow fine next door. There's a large maple that dumps seeds.
The main end goal crops:
Berries, paw paw tree, fruit trees, tough straight coppiced lumber for handles, snowshoes, etc.