As others have said, intervals and hills.
I was in your exact same position this time last year. I likewise have time constraints, my commute to work is 1 hour each way and I work 10 hour days so 12 hours. Also to muddy the waters some I got Lyme disease, was diagnosed first week of September so that took me off the bike for about 8 weeks, right in time for the winter shutdown.
So again intervals and hills. If you have hills locally get some times (use Strava or other app?) and try to do better every time you do the same hill. I was riding a hill close to my house last fall it took at least 10 tries before I finally was able to ride up it (8% grade, 0.7 miles long) without getting off and walking. But my first time making this was just about the time the leaves were turning color and I hate the cold. But I didn't want to lose what tiny little progress I had made so went searching for a solution. The solution had to accomplish two things, first allow me to train at odd hours when I'm not working and doing other family things and second keep me off the ice and snow and warm inside the house.
To cut to the chase what worked for me and my schedule was an indoor fluid trainer. At the beginning of last winter I couldn't hang with an easy 25 mile group ride on even small rolling hills. So I set my mind on using the winter off-season to really get on it. I put a smart trainer in my family room, got a subscription to Zwift and made a commitment. The first week on Zwift was just as hard or maybe even harder than outside group rides. Zwift puts a lot of stats in front of you and there are group rides and so forth so at first my ego and legs were still in the toilet. But I did make a commitment and drug my sorry butt downstairs and logged in and rode. Rode more and again some more, between mid November and mid May I put in over 3000 virtual miles and climbed about 270,000 virtual feet.
This was just as as painful and difficult as my group rides but I did it on my schedule and during the off-season. The result: Little old Tom is now smoking guys that only last year smoked me. I have completed 6 solo metric centuries so far this year and many other group rides. I do multiple laps around the hills I walked up just last fall. A hilly 25-30 mile ride is just a thing I do with no fuss, I just do it. Everything about bicycling has changed for me.
I'm not trying to sell you or anyone else anything I'm just repeating what worked for me. I wanted to exit last winter with some real riding skills and endurance, I did that and more, way more. And I did it on my schedule. The main thing is I stuck with it and because of all the hard work, time and pain suffered I don't want to lose it so it just feeds on itself and when I achieve one goal I add a new one to the list. You don't have to do it on a trainer and using a virtual program but if you have time constraints and you want to get a lot better you need to put in the time/effort. An indoor fluid trainer was perfect for me, it allowed my to get on the bike 5 or 6 days a week, every week whenever I could find the time and regardless of the weather or available sunlight. But again, intervals and hills, one way or another.