I don't think you have to hide the numbers for what you are doing, but rather just how you get them.
I've made my game more player facing, i.e. they players roll all of the dice. But I've also made it more narratively driven, i.e. the players don't invoke their powers, instead, they describe their actions. (Taken liberally from *World games)
I did this in response to exactly the problem that you're having- that DFRPG can be overwhelming at times. So how does this work?
The first part about the GM not rolling takes a lot off of the GM's shoulders, and makes things faster. You then use this savings to spend the time to interpret the players' actions, and trigger them based on the narrative for the players that are less familiar with the rules. Rotes help a lot with this.
So, the only things you have to explain to your player are- the rotes that you help them design with their characters, and the meaning of overcasting vs standard casting. Once you've done that, the player describes what they are doing, i.e. casting their rote, or putting more effort into it or really pushing themselves. You look at the numbers behind the scene, and give them the target. They roll... and describe what they are doing to invoke their aspects as needed, since they know what they need to make the roll.
This way, they are eased into the aspects of the game that have to do with Fate, while the crunchy things of magical manipulation are kept behind the scenes.
In summary, let them describe it in the narrative, you do the heavy lifting, then let them roll against a static number, invoking aspects as needed to make the roll if they want to so with more narration.
I know you're not a native English speaker, but in many circles of general nerd culture in America and on the Internet, there is a phrase for what you're doing right now: your spaghetti is falling out of your pockets.
In all seriousness, though, you seem very anxious about your situation and you should take a step back and relax. You say you're good at improv, but it looks like you're already sweating bullets because your party got sidetracked instead of getting to the quest you wanted to take them on.
I'm going to put my direct answer to your question first and some general advice afterwards.
Your players probably won't miss their characters. Why? Well, you generated them, and even wrote up their backstories. These characters were never made by the players playing them, and while they might have grown attached over a few moments, I find it extremely unlikely that they'll shed tears over a party that lasted for four sessions... that they didn't even make themselves. Even if the characters are extremely cool and interesting, that element of personal attachment just isn't there, which allows the players to truly say that they helped create an exciting story with interesting characters. This obviously varies from person to person, but again, I find it very unlikely that your players would have grown very attached in this case.
Go out with a bang. You know that amazing epic encounter you were saving for the climax of the plot arc? Yeah, run it now. You'll have to make some tweaks because you obviously aren't quite there yet, but get them there as fast as possible. Feel free to kill off PCs or even have a TPK at this point; memorable deaths are often much better than "and then they lived happily ever after."
Here's a few things to keep in mind when starting your next campaign:
Your content will come to light eventually, and it will be good. The quests you've designed will always find a way to come forward. Even if your plot arc is entirely ruined by something the players did, you will be able to recycle the content you made but never ended up playing, and I encourage you to do so for your new campaign. The only things that are truly lost are "hard" materials, like NPC stat sheets, etc.
You're in control, and therefore, you set the tone. Sometimes, it is best to take a page out of Gygax's book; after all, this is your campaign, and you put a lot of effort into it. Obviously you shouldn't take the entire preface from the AD&D DM's Guide to heart, but there is a point where a DM should draw a line in order for there to be some kind of structure, assuming you want your campaign to go anywhere. If your players are goofing around and killing NPCs for no reason, or making light of important people in-character, then they should be ICly punished for it; reprimanded for insulting a nobleman, pursued for attacking innocents, etc. It is also very possible to play a serious game in character and laugh until you're blue in the face out of character. This frequently occurs in the Dark Heresy games that I've played and ran.
You had better get used to murdering your darlings. This is a phrase commonly used amongst writers and creative designers everywhere in the U.S. The phrase means that you'll have to scrap ideas frequently, including ones that you really, really liked, so you had better get used to it. The saying is intended for use in the writing, film, video game, and other industries where a publisher or producer oversees your work, constantly telling you what can and can't make it to the final product based on time and expenses. However, it works just as well for when your ideas can't make it to the game because your players did something insane. And, on a related note...
Plan less. I don't know how much effort you're putting into writing everything ahead of time now, but you might want to ease up on that. From what you're telling me about your role-playing experience, it seems like you've been playing in a "safe" and slow environment where you rarely, if ever, have to scrap or re-do material. This happens literally all the time in regular tabletop RPGs, thanks to the insanely unpredictable nature of 4-5 different people working together. It will save you a lot of anguish if you lay out a basic outline of what's going to happen and then add the details once you're sure the players will be arriving there next session, or maybe two sessions later.
It seems like you've learned a lot already OP, which is great, but scrapping a campaign after 4 sessions (and while your players are all enjoying it) is something you should really avoid. If everyone else is having fun, consider either shaking things up a bit and changing your own notes, or coming up with a way to set them back on track, which doesn't always need to feel contrived or railroad-y.
EDIT: Well, now that SevenSidedDie has made that edit to your post, there are a couple of details that I didn't quite catch before, no offense. Since you said you have a month between each session, it seems like you might be over-planning because you have a lot of time between sessions. Heck, you might even consider having more frequent sessions, if you can't stop yourself from overthinking it in the intervening months. If in-person is not an option, use Skype and/or Roll20.
Best Answer
There are a few "next steps" that you can attempt -
4th Edition D&D is, in many ways very, very similar to the D&D Adventure System games. I recall seeing some threads on the WotC forums to this effect, and http://www.dndboardgame.com/ appears to be an effort to convert some of the earlier Adventure System games to that version of D&D
You could try one of the similar boardgames such as Descent, which has a richer character advancement mechanic than D&D Adventure System, and has a "Dungeon Master" controlling the monsters. However that would be a potentially expensive investment, and doesn't really bring you much closer to an RPG experience.
You could try an RPG like the Warhammer Fantasy Roleplay - https://www.fantasyflightgames.com/en/products/warhammer-fantasy-roleplay/ - which puts less emphasis on a pencil and paper character sheet, and rather has a feel closer to the Adventure System games with cards and other tactile elements.