Nathalie, you just find a pace that feels comfortable. Depending on how hard the previous day was will depend on how comfortable you will feel. I also like to put in little efforts to kind of activate the muscles. So if it’s available I will do the Richmond flats and once warmed up just push on the sprint.
Well don on doing the FTP test, they’re not nice, but at least now you have a benchmark.
FTP tests are horrible & intentionally so.
But they are essential for finding out a range thats comfortable and more importantly, do-able for you. Once thats set and you have a HR monitor I would go for workouts and just take your time building up.
Remember the rest days and listen to what your body is telling you. Not neccesarily your head as that will cave in way before your body will.
With regards to trainer difficulty, as others have said its not really difficulty. The same amount of power is either generated or required, only at a different cadence likely resulting in you having to change gear.
I dont want to waste my time changing gear when training for fitness or un-neccesarily wear through my entire drivetrain & thus have ran 0% difficulty (or as close as the slider lets you) for years now.
**Edit
Should mention this is on a Gen 1 Tacx Neo since 2018. Before that on a Tacx Vortex I was running 20%.
with regards to cadence during climbing, this mostly comes down to individual preference.
however scientifically, spinning away in an easy gear at a high cadence (85rpm++) is technically more efficient as you are using less of the powerful fast twitch muscles which generate a lot of lactate. so if unsure start out seated & spinning.
some people however prefer to cycle up climbs out the saddle at ~60rpm aka. grinding it out.
personally this doesn’t really work for me as i am predominantly fast-twitch and as soon as i activate those muscles my lactate goes through the roof. it is very useful for a short intense burst of power, but doesn’t work out too well on any sustained climb that lasts over 30-60sec for me 
also - if spinning away at 90rpm - it can be useful to shift down 2-3 gears and get out the saddle for 5-10secs (without drastically increasing power) just to get a bit of a break from being in the same position for a prolonged period of stress. can sometimes help flush out a bit of lactate too 
i dont get why you would run a £1000 smart trainer on 0% trainer difficulty, as it means you wont experience a difference in gradient/resistance at all, so you might as well be riding on a £200 dumb trainer xD
changing gear may be a waste of time, but if you cycle at the exact same resistance/cadence for >90% of ur training, you will get very good at riding at that intensity, but feel very uncomfortable when it comes to riding outdoors in a group where you dont get to set the pace. i like it just to force me to ride under a variety of different conditions, although i have to say i never really run it above 50% because changing multiple gears every 2 seconds for every tiny change in incline is a bit of a ball ache.
Depends what your after in a smart trainer I guess.
My original dumb trainer, a cyclops mag, did me fine for a long time. However with zwift comes the need for ERG mode and the output of power and cadence.
For me those are the only things I need or want to see and the only reason I bought a smart trainer.
I have no particular interest in simulated gradient effects on my geartrain or a modern one, steering. But then I dont take part in much group riding with my only planned events (real world) being TTs, Duathlons or Tris so as you say, reacting to group riding issues have never been a problem for me though I might change difficulty if I began to take part in such.