FTP and Testing. I think the Ramp Test is a fine way to get you into the ballpark for finding an FTP to set the intensity of your interval workouts. If a workout program is too hard, just lower your FTP a few watts and wait for the next retest to move it up.
I actually just use the FTP results zwift gives me from races. I find that number is fine for any workout I have done.
Power Consistency and ERG? Are you using ERG mode when you are trying to hit zones, or are trying to hit particular zones free-riding outside of ERG mode? For free-riding it is just practice to hit consistent numbers (though racing can train you to fluctuate to help keep position in the groups!). For ERG mode ... pedal at a consistent RPM and let the trainer take care of the resistance. And make sure you turn off erg mode smoothing in the wahoo app for the kickr core, so you get the real data in erg mode!
Trainer Difficulty Nothing wrong with putting it at 100% and leaving it there for a more realistic feel through the pedals on the steep gradients, but there are some good reasons to not put it there all the time. If I have my flat land cassette on the trainer, it really is not that fun to get punched in the legs by the 15% grades that Zwift sometimes hits. Also, in certain spots in the game (the esses or Titans Grove) the rapid changes in grade happen just slightly behind the avatar on the road, and if the grades are less sharp there is less of a mental disconnect between legs and visuals. Also, a low trainer difficulty is a great way to reduce the amount of shifting you have to do because shifting can be loud and annoying indoors. For me though, since most of my zwifting is done with an eye towards zwift racing, I don't really feel like practicing shifting for outdoors, I lower the difficulty quite a bit. I actually race most of the time at ~30%. I find that is a good number, since you can still feel even small changes in gradient in the pedals, and you rarely have to shift over the tops of climbs, making it easier to stay with the group at the most important points of races. I do like to race Alpe de Zwift at ~80% or even 100%, but that is because the grade on the climb is reasonably similar the whole way up and I like the way the little ring feels on the steep stuff. Also, I do sometimes feel the need to practice shifting, so I do free ride at 100% sometimes.
Sucking Less For this, I just race a lot on zwift, and ride around unstructuredly at other times. Both indoors and outdoors. Not the most efficient way to improve, but very enjoyable. Also, I can say that there really is skill involved in zwift racing even if it does not look like it most of the time. There are only a one variable you get to manipulate, Your Power, but timing that power matters, and reading the other riders and the gradient are really important to keep the virtual speed up. It is not that hard to learn, but you can really save a lot of energy with good racecraft. Really a better answer is ... I Have No Idea.
Racing and Such Opt-In to Zwiftpower.com if you have not already. Definitely better results and data there than given by the Zwift app or the Companion app. And a good event list is there as well. Your race result sounds great, and a good place to start. Just keep racing! Also, there is definitely a large gap between all of the categories, and you found that with that group ride. For most upper B's 3.4 w/kg is that nice pace where you get to recover in the race. Constant 3.4 w/kg is hard work for the C's! For other group rides, I think some of the ADHR rides are closer to 3 w/kg, but I can say I do not do many group rides, and most of the ones I have done were 2.0 w/kg rides on recovery days. That event list on zwiftpower would be where I would look for a particular type of group ride. I guess the problem normally is having the group ride be at the right timeslot on the list!
One of the best types of events you have not mentioned are the Fondos like the El Giro de Rigo last weekend (or the Zwift run Tour/Stage/Races). These are mass start, normally high participation, events that are really races, but everybody might not be racing and they are not categorized as real races on zwiftpower. In them, the large starting pack breaks up just by natural drafting physics and you ride the whole event (I have done up to 80 mile Fondos) with a bunch of people who are all about your level riding in a group. Plus, if you are dropped, there is always another group behind you can latch onto!
I can say though I really enjoy racing. When I started on Zwift I was just over 3.2 w/kg FTP@70 kg, so not only did I have to move to up to B cat right away, I was still getting dropped from the C lead groups because of poor racecraft! It took a couple of months of racing when I had increased my FTP to about 3.6 to 3.7 w/kg before I could hang with the main B pack. (And it took about another year more to get to just 3.9 w/kg!). Entering a few races a week and practicing hanging onto the main B group for as long as possible really helped ingrain when and where I had to put the power down and when I had to conserve energy. Now 150 races later I am ranked in the top 30 for B's on zwiftpower. Still not an A though.