Sounds like you were doing a fair bit of good quality training already, even if it was unstructured. Were you tracking mileage previously, and if so how much of an increase (if any) is going to 2500 miles/year? Also you say you are doing lots of SS work - are you training the higher intensities as well? SS really builds your base in preparation for increases in FTP, wouldn’t necessarily expect to see a big ride in FTP from doing SS work unless you were starting from an untrained state (you weren’t), or it was a big increase in training load (see previous question). MTB, club rides and commuting (if you’re like me and like racing cars and other commuterd anyway…) all involve a lot of work at threshold and above, so if you’re missing that from your training then that would also explain it.
Second point is that there is much more to bike fitness than FTP tests. E.g. I did a lot of SS and endurance work in the last 3 months. My FTP test results (using ramp test) haven’t increased much, but my aerobic base has increased hugely. I can ride at or just below FTP for much longer durations and at lower HR than I could at the start of the year, I’m also recovering much quicker from hard efforts than I was previously. Those are useful improvements, and more importantly indicate that as I move into Build and start doing more Threshold and VO2max work then I should have the base to make some big gains.
And lastly, 5000 miles in 2 years really isn’t that much, it’s only 50 miles a week. Not sure how your trainer calculates speed/distance, but that implies you’re training about 3 hours/week on average. The benchmark for somewhat serious riders that I know seems to be more like 5000 miles/year - that’s people who get out for a club ride most weekends, supplement with a few midweek rides and don’t take big chunks of the year off. The serious people who are riding most days are doing more like 10,000 miles.