As background, I've hiked the Kungsleden in 2014 (my second hike, and first self-organized), as well as both marked trails and cross country in Norway and Iceland. To add to the risks already mentioned:
Do not underestimate the general, less obvious hazards in tough terrain. For example, both in Iceland and slightly off trail in Kungsleden I encountered overgrown streams. What looks like a mossy path between thick bushes (and easier to walk) might turn out to be hollowed-out gound, and stepping on it can end half a meter lower than it should have. That's something you can't really prepare for just by reading online articles, you learn from experience.
If someone gets injured by something like that, a remote area without marked trails could become a serious problem - even if you have a satellite phone, describing the position while inexperienced with navigation could be hard. On a marked trail, that's a lot easier, mentioning the previous and next hut/waypoint makes you easy to find.
Advantages of a marked and maintained trail:
- no (or easy) fording (less risks, no fording shoes necessary)
- hazardous terrain is mostly laid out with planks, eliminating a lot of less obvious risks
- navigation (even with basic map/compass skills) is easy, as well as communicating your position in case of emergency
- others will be around to help out if necessary - both hikers you meet on the road, and hut staff
- in case of bad weather (which can happen often and quickly) there are some huts to take shelter and dry your things. Hiking in wet
boots for a week is not really pleasant.
As you are inexperienced and have a lot of things on your mind - save yourself a lot of trouble and risks, take a marked and maintained trail for the first hike. You have less to prepare for, less that could go wrong, and there are others around who could help out. The Kungsleden is still very beautiful, there's a reason it's famous.
As for how crowded the Kungsleden is (or was 2014):
Make sure you don't go there when Fjäll Raven Classics happens. That's hordes of good-weather tourists going from Kebnekaise to Abisko in a short while, definitely not enjoyable. Besides that time, it's not that frequent you meet someone.
We went from Abisko to Vakottavaare, and after we got south of the Kebnekaise, it got a lot quieter. Might be useful for you - someone around for the first days, in case something went wrong, then less company when you've got a hang of it.
Even if that's still too crowded, take another trail, there are more choices in the region, like Padjelantaleden. But going off trail for the first hike seems reckless to me. Once you got the basics covered, you can aim for the more remote areas.
As side notes:
Even on Kungsleden, there was no cell phone coverage except for the Kebnekaise basecamp area. Satellite phone might be nice to make calls, but for emergencies, a Personal Locator Beacon (PLB) might be better. I'd say it's overkill for the Kungsleden (see navigation), but off-trail it should be essential.
For the training hikes, include some steep ascends. By my experience, that's where boots fail first. Do it every two weeks with full weight, and your body gets used to it, that makes it a lot easier on the hike.