While the focus on a metric like average speed has it’s own debate, there’s been some good ideas thrown out here to help you find a way of bringing it up on rides where you’re not pushing it hard (i.e. zone 2).
Perhaps you’re looking at the marginal gains aspect of improving your average speed (the icing), when you likely need to simply keep focusing on improving your fitness (the cake). I had a look at your profile and I have to say that you are an excellent and dedicated cyclist. Serious kudos for your consistency and you’ve stuck to it.
You’ve also been been bumping your FTP manually for quite some time. When was the last time you did a ramp test or 8/20min test to assess your FTP?
Can you provide some details on your equipment set up? What power meter are you using indoors and outdoors? What trainer are you using indoors? I noticed you don’t record HR very often, and this can sometimes help a lot in understanding how hard you actually are working and is it aligned with your Zone 2 power.
I am however remarkably baffled by your average speed given your level of fitness. Please don’t take this the wrong way, I’m only trying to provide insights into helping you get faster, but your average speed on outdoor rides is astoundingly slow for someone with an FTP of 4.8 W/kg. You realize your FTP places you comfortably as the same level of fitness of a Cat 1 racer and close to domestic professional riders.