I'm another PNW rider, but I dislike riding in the rain and getting my bike dirty, even my rain bike, and doing all the laundry associated with rain riding. So in winter, I ride outside once a week no matter the weather, but the rest of the week I ride my rollers in what is basically my mostly unheated garage. I don't use a PM. I just have my HRM and my resistance rollers on which I can do most anything I can do outside except ride for over 1.5 hours, which is my inside limit. I don't find the rollers terribly boring because if I don't pay attention I come off and I don't like that.
I train for one late summer event, and I advise doing something similar. It's hard to get faster if one does the same thing all year, besides as you say, it's boring. Having an event (or a non-event) to peak for encourages one to periodize one's training, meaning I'm constantly changing what I do on and off the bike - I also run, ski, and go to the gym. I think a goal is helpful, say some local double century or some similar thing of your own devising. I have a number of difficult training loops which I work toward being able to complete every summer.
I use TrainingPeaks Premium which gives me numbers to strive for and an annual training plan. Within that training plan, I devise a progression of bike and gym "exercises" if you will, which changes from month to month as I work toward my goal. Some things I do all year, like recovery and moderate rides and high cadence drills. Other things I gradually introduce as I become fitter. So I do the usual base stuff, then zone 3 work of various sorts, then zone 4 workouts, then in summer add some sprints and speed work, though at each level I retain workouts from the previous level, just not as many. One can't do only zone 3 or zone 4 workouts.
__________________
Results matter