I'm no expert by any means but I've come to something of a crossroads myself, having pretty much 'peaked' after 9 months or so of reasonable gains.

I'm approaching the next phase by splitting my time between riding and training. Riding is going out as you've been doing and working on pace, technique, spinning etc, and trying to get my lap times down. 'Training', for me, is taking the bike to steep hills and doing some up/down reps. I don't particularly enjoy it, but like you I don't find myself getting breathless when I'm out on a ride, even when I push hard. Working on hills is a pain but I'm hoping I'll start to see some benefits on my rides, which is what it's mostly about. I'm also taking the MTB to trails where I can get a more varied type of workout than just riding around on the road bike. The MTB helps develop different parts of the leg muscle and works on the upper-body muscles too. It's also a lot of fun - that might be something you could look into.