I am not training using TR so I hope I am not going against their principles with what I‘ll say now, but miles/ hours/ speed/ average watts etc are a very poor predictor of whether you have done everything to improve.
Over the winter (understanding that you will not participate in any events over the next 5-6 months), it makes a lot of sense to plan out how many hours you can put into training. Given that’s 6-8 hours, I would plan 2-3 endurance based rides, where you don‘t go into threshold, and then do 2 hard, cycling specific gym work outs. All your intensity should come from these early on, and focus on progressive overload on these gym sessions.
As we go deeper into the winter, substitute one of the gym sessions with an intense session on the bike and progress on that. Then a month or two later, get rid of the next gym session for another intense cycling session.
I‘d really focus on progressive overload, a principle many people forget. They ride and ride and ride and wonder why they never get better. Much of that can be attributed to „not even testing if you got better“. You might incorporate an FTP test every 4 to 8 weeks over the winter, to see if things are progressing as they should. If I may recommend one of the best and most comprehensive cycling training channels on YouTube, go and look up Dylan Johnson. This video should help you out and explain in much better detail, what I just tried to convey:
Fast on 6hrs
Fast on 10hrs weekly training:
There are many ways of doing it, like the (in)famous sweetspot training, that as I understand, TR is working on a lot. I can say, that I had great „success“ with training like it is explained in the videos. I started cycling in February of 2019, and did some riding around (as I understand similar to what you do) for 8 months. I did workouts based on GCN videos and chased local KOMs and jersey on Strava. I got myself to just under 4W/kg doing that (I was pretty obsessed with powerlifting before cycling and therefore reasonably fit). Over one year of very disciplined training based on these principles I got myself from a best 20mins 312W at 74kg (October 2019) to a best 20mins at 372W at 71kg (October 2020), so at least for me, these principles have worked wonders.