If the handout for an intangible thing is another intangible thing,
This is the crux of my problem with this suggestion, and the only thing that really matters. Missions aren't an intangible thing - they're an interesting little extra option in gameplay terms.
The people who play this game seriously "caring about lore" will be few and far between. And out of the ones who do, they're not necessarily interested in playing missions - once you've read the mission introduction, you don't need to actually play it out: no more lore information is gained from that. You play it purely because you'll have fun playing it. And there's no advantage in denying people that option.
You call a mod to unlock it a cheating mod. You say that flying to every planet for a tangible reward would be unenjoyable and a bother.
So what if I find some people discussing some cool mission and different ways to play it, so I decide to open up my game and play it. Woops! You have to either load into your game and spend 10 mins flying to some arbitrary planet or download an unlock mod. Why is that an advantage?
I have no problem with exploring the world unlocking lore info. It sounds like a nice idea, in fact. But missions are not "intangible" and I have no idea why you'd claim them to be. It's not you unlocking some text or a paint job. It's something you might want to do for fun for completely unrelated reasons.
I see no reason why the people who want to play missions would be the people who want to explore. In fact, to me, those two groups should be somewhat separate by nature - missions are a good way to get some playing done for people without much time to play, whereas exploring the lore could only really be rewarding if you're spending a good deal of time digging into it.
Since your definition of "explore" seems to just be "click a bunch of preset locations once" why do you consider a mod that circumvents that cheating? While it's hard to really call anything cheating in a single-player game, that seems like an example of skipping something boring and tedious. I can think of various games that had a reward that I wanted for checking a bunch of locations - even at maximum efficiency: running straight from each one to each one with a guide, I still found it a profound waste of time.
But again, I'll bring it back to a single point. What's the relationship between "visiting every planet once to read the lore" and "playing missions"? I visit lots of planets without being concerned with lore (having fun in the process), and I've played plenty of missions (having fun in the process), and often don't read the briefing. How is interlinking these two things improving the game, for me or for anyone?