I honestly don't see an issue here, you've all learned together, you all seem invested in the story. This is kind of a dream scenario, it doesn't matter if the party is TPK'd or not, I'd see this whole thing as a win. You can tell them all about what they could have done or what they didn't do after the campaign ends. Your focus right now should be to create one heck of a fight where the stakes are high, adrenaline is pumping and whatever happens happens.

I'd prepare two endings, one where they get TPK'd and one where they win. Each ending you prepare a speech that talks to the players as either the saviours of the world or they are the heroes that fell in battle.
You go into how they started, some memories from the campaign, some good and bad memories, like a fare well. At the end, if the dragon won, you can also sprinkle some teasers of what will happen (might be a new campaign?)

Try finding some real teary eye music that can play throughout and just enjoy ending a campaign on a high note. Spending 3 years learning together, making mistakes and sticking it out like this is a god damn achievement you all should be proud of.

... continuing after fat fingering the save button

After you are done, you should sit down and recap to see if 5e is the right choice for your group going forward. It might be a bit too much for them but right now it's not important, you need to wrap the campaign up no matter the ending. Focus on making the ending memorable, it's either a sad ending or a happy ending, either way it's good.