Another test ride. Today I kept the slope flat to see what kind of power I needed to maintain 25km/hr - turns out just over 100 watts with no wind, which I found surprisingly low.
I did some more power sprints of 15-20 seconds. The first one I thought I had maxed out at 845 on flat, but then I found a bit more and stubbornly pushed over 1000 (ok 1002) which translated to 60 km/hr on the trainer. The problem was that my heart rate stayed elevated after the big push and had a hard time dropping back below 150 even at a very relaxed power level of 100-110.
I suspect my FTP will come in low to start the winter, probably the price I pay for not having many hard miles this year.