Hey @Chuck_Tatham,
Good question here!
As a northern midwesterner, I have a similar yearly schedule to you, it seems.
Fat biking is pretty big where I live, but I’ve had a hard time embracing lugging around a big bike in the cold and maintaining it when things are frozen. 
In the winter, I prefer running and using my indoor trainer. Because of this, my fitness usually peaks in the springtime.
It can be hard to balance riding for the love of it and constantly looking for fitness improvements, but I manage to do this as best as I can by simply ensuring that I get a good mix of ride types in as often as possible.
I live between two great MTB trails, and I can ride to either one in around 20 minutes. Depending on which one I choose, this usually consists of either paved bike paths and snowmobile trails, or bike paths and low-traffic, slow-speed roads, which are great for intervals.
I’ll often use little segments on my commute to the trail to do 10-15 minute threshold intervals, then get my trail laps in, and either head home or ride a bit further on the road/path to get in some more work on tired legs afterward.
That type of work isn’t all for nothing. It’s not as productive as pure structured training, but it’s still training stress. If you’re looking for a better way to turn MTB trails into a more structured ride, I’m not sure how possible that is at the moment. 
My advice is to ride as much as you’d like, try to get some variety in efforts in (get some sustained work if you can in conjunction with the punchy nature of MTBing), recover well, and enjoy the ride.
It’s all about enjoying the process. 
It’s not super realistic for most of us to be 100% focused and productive year-round, and if you’re getting trainer rides in over the winter, I wouldn’t hesitate to use some of the summer to enjoy your time on the trails. That’s what cycling is all about!