I agree with others, I feel like I can put out more sustained power more efficiently on my road bike than MTB, independent of aerodynamics or rolling resistence, etc… The configurations are designed for very different things. @David_Strappazon if you were to put an inexpensive road bike on that trainer you’ll find you can perform at a higher level in Zwift and have higher gears to ride faster for short sprints. I am using my 25 year old part aluminum, part carbon, 7-speed road bike on my wheel-on trainer (7-speed cassette won’t fly on higher end trainers). it is fine as I have a smaller cassette to get me the higher gears when riding, and when doing workouts in ERG mode your gear makes no difference.
To that point, you mentioned shifting during the FTP test. If ERG mode is working correctly, your gear should make no difference. Shift all you want, the trainer is going to hold you at the specified power. Spin faster, it relaxes the resistence, spin slower it cranks it up. Don’t shift, it has only a momentary fell good effect then the trainer is going to normalize back to the specified power.
As David and Tim point out, 2.5 w/kg is respectable for FTP, specially for a new rider. If you had a road bike on the trainer and did a new FTP test with what you know now, I’d bet it comes up a little. But as noted, don’t let FTP rule your life. You are who you are. No, you and I can’t put out 5 or 6 w/kg like the young stud racers, so what. We are riding at our level for enjoyment and training. I’ve been riding in the Colorado mountains for 35 years and am in well above average shape for a 58 year old (i.e. I leave all my friends in the dust) and am only 70 kg, yet my FTP is only 3.3 w/kg. I never had the racer muscle makeup and certainly won’t get it at my age. Use Zwift to improve your fitness over the winter, and enjoy riding indoors.